Wow, this is a good question. I dove into the search tools on LDS.org and came up with a few things that you've probably seen already, but they were instructive for me.
It looks like the reason for death has directly to do with circumstances of the Fall.
In Alma 12:23-24, it says:
23 And now behold, I say unto you that if it had been possible for
Adam to have partaken of the fruit of the tree of life at that time,
there would have been no death, and the word would have been void,
making God a liar, for he said: If thou eat thou shalt surely die.
24 And we see that death comes upon mankind, yea, the death which has
been spoken of by Amulek, which is the temporal death; nevertheless
there was a space granted unto man in which he might repent; therefore
this life became a probationary state; a time to prepare to meet God;
a time to prepare for that endless state which has been spoken of by
us, which is after the resurrection of the dead.
So death must come, among other reasons, because God said so. Is there yet a reason behind that? I don't know. There's certainly lots to learn from death that may only be accomplished by having death. For instance, how does knowing that we will die affect how we live?
Or, would it fair to say, that since Adam and Eve became even as gods through the Fall, knowing good from evil, and comprehending opposites, that life/birth must have its opposite, being death?
We've also got Alma 22:14:
14 And since man had fallen he could not merit anything of himself;
but the sufferings and death of Christ atone for their sins, through
faith and repentance, and so forth; and that he breaketh the bands of
death, that the grave shall have no victory, and that the sting of
death should be swallowed up in the hopes of glory; and Aaron did
expound all these things unto the king.
Although we aren't all translated, if we're righteous, the sting of death is swallowed up in Christ (1 Cor. describes the sting as sin).
Besides, if all people were just translated/taken up instead, I think that kind of death wouldn't cause us to search for something more to life. Dying has a purpose in urging us to consider our state of being and our standing with God, or even to search for God, while we yet live. This thought is mostly philosophical, but even prophets use this reasoning to convince us to "consider our ways," (Haggai 1:5) and "turn again to the Lord" (Lamentations 3:40).