From wels.net (emphasis mine):
“For Jesus’ sake” means that we are coming to God in prayer, not pleading our own merits, but (“on account of”) Jesus’ merits... When we end our prayers with “for Jesus’ sake,” we are acknowledging that Jesus is the one who has bridged the gap between a holy God and sinners. “For Jesus’ sake” has the idea of asking that our prayers be heard and answered, not because of who we are but because of who Jesus is. And of course whether or not we use that expression, that thought is foundational to all our prayers.
From Truth Magazine (emphasis mine):
FOR CHRIST'S SAKE hath forgiven you" (Eph. 4:3-2). The Holy Spirit said God hath forgiven us "for Christ's sake." This being true, why should it be wrong for us to use the phrase "for Christ's sake" in our prayers? Certainly, certainly, ask the prayer in the name of Christ (Rom. 1:8, Col. 3:17). But in view of Ephesians 4:32 may we not use the words "for Christ's sake?" Does someone say, brother Plum it is not for Christ's sake we pray, it is for "our sake," "my sake." I understand Christ was perfect and we are not, and personally I do not feel worthy to say to my God grant my petitions "for my sake," do you? I need help, forgiveness, all right, but what is there about me to ask the Lord to do it for my sake? Nothing. Absolutely nothing. But when I ask God to have mercy upon my requests for Christ's sake, that adds weight to my petition. Shows our knowledge of our unworthiness, and shows our belief in Christ's worthiness.
From the Catholic Encyclopedia:
the Council of Trent emphatically declares (Sess. VI, cap. xvi, in Denzinger, 10th ed., Freiburg, 1908, n. 810): "the Lord . . . whose bounty towards all men is so great, that He will have the things, which are His own gifts, be their merits."
Since the Catholic Church believes that some merits are given by God to be a man's own, does it follow that "for Jesus' sake" is NOT "foundational to all [his] prayers?" And, if the phrase is not foundational to Catholic prayer, when (if ever) is the phrase acceptable in Catholic prayer?