Definition
Accommodation means that we use finite terms to describe an infinite God, and thus naturally come up short. This is certainly true, so in some sense every description of God is an accommodation. However, that does not mean a given description does not correspond to a truth. Thus, for the purposes of this answer, I am restating the question to be "What is the basis for believing descriptions of God's wrath correspond to a true aspect of His nature?" which I think is the real question here.
Basic principle
Perhaps the strongest argument against viewing God's wrath as an accommodation is the lack of evidence for this understanding. The view is primarily driven by human desire to avoid putting "impure" emotions such as anger on to God. However, the idea that anger is "wrong" is driven my philosophical ideas about how the ideal person should act, not the Bible. Indeed, Paul writes:
Be angry and do not sin; do not let the sun go down on your anger, and give no opportunity to the devil. (Ephesians 4:26-7, ESV)
Notice what Paul does not say here - "do not be angry." Instead, he says something like "It's OK to be angry, but resolve your angry quickly, so that it doesn't develop into something actually sinful." This line is actually an illusion to Psalms 4:4:
Be angry, and do not sin; ponder in your own hearts on your beds, and be silent.
So, if anger is not automatically sinful for man, then there is no need to "explain" God's anger. And there is a real danger is seeking a way to explain away things that make us uncomfortable. If "negative" emotions attributed to God are merely accommodations, who is to say that "positive" ones aren't? Perhaps God is actually ambivalent to humanity and we just think He loves us because that of our finite understanding of God... It is one thing to say the "eyes of the Lord" (e.g. 2 Chronicles 16:9), which can easily be read as a metaphor in context, are an accommodation. It is quite another to say something (e.g. wrath) that is explicitly described as an attribute of God is one.
Finally, it should be noted that while "wrath" a synonym for anger, it is not equivalent. Biblically, God's wrath is the opposite of his satisfaction or pleasure in men's actions. That is, it is a term used to describe His displeasure with man. It is described in terms of anger, sadness, even hatred, but it is not merely an emotion, but rather part of God's righteous judgement. (e.g. Romans 2, see below)
I will now analyze some specific verse that illustrate that the "wrath of God," while perhaps not a 100% perfect description, is a fundamentally accurate way to describe an aspect of God. The list is by no means exhaustive.
Romans 1
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. (Romans 1:18-20)
Paul's argument here is that everyone is without excuse for knowing God because his actions are plainly seen, and from these actions his divine attributes are evident. To back up his argument, he says the "wrath of God is revealed from heaven." That is, evil and unrighteous behavior have clearly been punished. If God was not truly wrathful, and was only described that way for our benefit, then Paul's argument would have no force. The unbelievers certainly could not be expected to naturally perceive wrath that did not actually exist.
Indeed, the wrath of God is contrasted with "the righteousness of God [which] is revealed from faith for faith." (v17) While righteousness can only be seen through the eye of faith, wrath is apparent to all. One would have to jump through hoops to explain how the "unreal" wrath of God is apparent while the "real" righteousness of God is invisible to unbelievers.
Deuteronomy
A similar theme is found in Deuteronomy.
Take care lest your heart be deceived, and you turn aside and serve other gods and worship them; then the anger of the LORD will be kindled against you, and he will shut up the heavens, so that there will be no rain, and the land will yield no fruit, and you will perish quickly off the good land that the LORD is giving you. (Deuteronomy 11:16-17)
Beware lest there be among you a man or woman or clan or tribe whose heart is turning away today from the LORD our God to go and serve the gods of those nations. Beware lest there be among you a root bearing poisonous and bitter fruit, one who, when he hears the words of this sworn covenant, blesses himself in his heart, saying, 'I shall be safe, though I walk in the stubbornness of my heart.' This will lead to the sweeping away of moist and dry alike.The LORD will not be willing to forgive him, but rather the anger of the LORD and his jealousy will smoke against that man, and the curses written in this book will settle upon him, and the LORD will blot out his name from under heaven. And the LORD will single him out from all the tribes of Israel for calamity, in accordance with all the curses of the covenant written in this Book of the Law. And the next generation, your children who rise up after you, and the foreigner who comes from a far land, will say, when they see the afflictions of that land and the sicknesses with which the LORD has made it sick-- the whole land burned out with brimstone and salt, nothing sown and nothing growing, where no plant can sprout, an overthrow like that of Sodom and Gomorrah, Admah, and Zeboiim, which the LORD overthrew in his anger and wrath (Deuteronomy 29:18-23)
Here, we have two instances of a specific sin (turning away form God) leading to a specific punishment (barren land). The specific language makes it difficult (exegetically) to see this as anything but a literal description. Furthermore, this punishment is something that "all the nations" will recognize as God's wrath (v24). If wrath was a mere accommodation and not actual action by God, it is hard to explain how Gentiles will understand that God is in action here.
Romans 2
We know that the judgment of God rightly falls on those who practice such things. Do you suppose, O man—you who judge those who practice such things and yet do them yourself—that you will escape the judgment of God? Or do you presume on the riches of his kindness and forbearance and patience, not knowing that God’s kindness is meant to lead you to repentance? But because of your hard and impenitent heart you are storing up wrath for yourself on the day of wrath when God’s righteous judgment will be revealed.
... but for those who are self-seeking and do not obey the truth, but obey unrighteousness, there will be wrath and fury. There will be tribulation and distress for every human being who does evil, the Jew first and also the Greek (Romans 2:2-5; 8-9)
In this passage, Paul clearly ties the wrath of God to His righteous judgment of mankind. By doing evil, the sinner is "storing up wrath" that will be result in judgment on the "day of wrath." If the wrath being stored/delivered is not literal or not roughly equivalent to human wrath, then it is difficult to see what "tribulation and distress" the sinner will be suffering. Note also that the suffering is tied not an abstract person, but rather "the Jew" and "the Greek" - precisely Paul's original audience. If we were to understand this passage as a metaphor for God's character, we would expect at least some sort of textual clue, but instead the message is delivered in very concrete terms, and explicitly directed at the (original) reader.
Leviticus 26
Leviticus 26:14-43 describes the punishment Israel will face for failing to keep the commandments. Here again, specific sins are met with specific punishments, making it difficult to view God's wrath as merely accommodating language. The whole section illustrates the point, but I would like to focus one thought:
But if in spite of this you will not listen to me, but walk contrary to me, then I will walk contrary to you in fury, and I myself will discipline you sevenfold for your sins. (26:27-8)
These verses uses the doubled pronoun I myself to emphasize the personal nature of God's punishment. If, for example, man's punishment was just the natural consequence of sin and not a direct action of God (i.e. if God's wrath was accommodating language), this emphasis would be out of place. Furthermore, the sevenfold nature of the discipline would seem to indicate that the punishment is more than the inevitable consequence of the sin. The author of Leviticus is making it quite clear that the punishment given is from the direct action of God, which means it emanates from an actual aspect of God's nature.
Matthew 11
Then he began to denounce the cities where most of his mighty works had been done, because they did not repent. “Woe to you, Chorazin! Woe to you, Bethsaida! For if the mighty works done in you had been done in Tyre and Sidon, they would have repented long ago in sackcloth and ashes. But I tell you, it will be more bearable on the day of judgment for Tyre and Sidon than for you. And you, Capernaum, will you be exalted to heaven? You will be brought down to Hades. For if the mighty works done in you had been done in Sodom, it would have remained until this day. But I tell you that it will be more tolerable on the day of judgment for the land of Sodom than for you. (Matthew 11:20-24)
In Matthew 11, Jesus harshly rebukes a series of cities for failing to repent. In Matthew 23, He casts similar judgment on to the scribes and Pharisees. Although neither passage uses the term "wrath," it is apparent that Jesus is passing judgment on these groups. The passages certainly read much more naturally if one sees an element of anger in Jesus' speech. In Mark 3:5, He is explicitly described as angry:
And he looked around at them with anger, grieved at their hardness of heart
Jesus on Earth can legitimately be described as angry, and He describes judgment of certain groups in harsh "wrath-like" terms. As such, it is only natural that passages that describe God's judgment in heaven in similar terms be read as accurately describing a true aspect of God's nature.
Variety of contexts
Another reason to believe that the wrath of God is not merely an accommodation is that the descriptions of wrath occur in a wide variety of contexts. It occurs in history writings (Exodus 32), the Law (Leviticus 26), Poetry (Psalm 59), prophetic writing (Isaiah 13), the Gospels (Matthew 11), letters (Romans), and of course the Apocalypse (throughout Revelation).
Wrath is attributed to God the Father, Jesus, and the Holy Spirit (Hebrews 3). It is brought against Israel, Gentile nations, mankind as a whole, specific individuals, and the Serpent/Satan (Genesis 3). It is hard to find a context where God's wrath is not described in some fashion.
Conclusion
The idea that God's wrath is an accommodation stems from a desire to avoid the difficulty in seeing God as angry. It is often phrased in a condescending fashion such as "ancient people needed to see God that way, but we moderns know better" - either by believers to make God more palpable, or by nonbelievers in an effort to discredit the Bible.
The idea that anger is wrong does not stem from the Bible, however, but rather a man-made sense of the "enlightened man" ideal where man acts rationally and without emotion, or at least without negative emotion. This "ideal" is directly contradicted by the Bible and also carries unwarranted pride in one's own intellect. In reality, emotion is an essential part of human nature and logically an essential part of God's nature if we are indeed created in His image. Just as one cannot fully understand God's infinite, perfect love, one cannot fully comprehend His wrath either. That, however, does not mean that our terms to describe His wrath do not correspond to a true aspect of God. Furthermore, that His wrath is real is backed by straightforward exegesis. Those who wish to say otherwise must twist scripture to an extent that if applied elsewhere could make God's love just as "unreal."
