My personal opinion is that the vote was made primarily as a matter of perception than necessity. The perception among Catholic theologians was that Luther had given the deuterocanon a far lesser place than had been historically so. In one way I agree. That Luther placed the deuterocanon in a section separate from the rest of scripture by default gave the appearance of a secondary status that had not previously existed. On the other hand, Luther spoke of the deuterocanon in much the same way as theologians prior to him. Indeed, even Cardinal Cajetan, Luther's questioner, recognizes that the deuterocanon holds something of a secondary status, yet he still refers to them as canonical:
For the words as well of councils as of doctors are to be reduced to the correction of Jerome. Now, according to his judgment, in the epistle to the bishops Chromatius and Heliodorus, these books (and any other like books in the canon of the bible) are not canonical, that is, not in the nature of a rule for confirming matters of faith. Yet, they may be called canonical, that is, in the nature of a rule for the edification of the faithful, as being received and authorised in the canon of the bible for that purpose.
This language is similar to the language that Luther uses, but I believe his setting the books apart from the rest if scripture reduced them to a status that is not in keeping with the historical canonicity of the books. Long and short, I believe the deuterocanon was and is canon, even as it is not as authoritative as Jerome's canon. Perhaps that's the reason there wasn't a hard "yea" vote.
I probably am not far from you in my opinion of the so-called deuterocanonical books. The bottom line is that there has always been doubt about their worthiness to be considered part of the Old Testament Scriptures. Your position seems to be, and if I misstate it, please correct me, that they are to be considered part of the Old Testament Scriptures, but not with the same status as, for example, Exodus or Isaiah.
Luther readily agreed that the content of the better of them, for example, the Wisdom of Sirach, 1 and 2 Maccabees, was very valuable both historically and theologically, though more so historically. I think this is right. Whoever wrote them did so in order to bring glory to God, and not to put forth a different theology. And they should be read and honored in that light. However, having read them pretty thoroughly, both in English and Greek, I really don’t think their authors had the idea that they were inspired in the same sense as the writings of Moses or Isaiah. In fact, there seems to be real consciousness that prophecy of that sort had not been present in Israel for a very long time. Take a single example, 1 Maccabees 9:27. It is difficult to read this without drawing from it the conclusion that the author knew that no prophet sent from God had appeared in Israel up to his time, and that he was making no claim himself to be such. And there are other passages that give the same impression.
I think it is very unfortunate that the deuterocanonicals are not included in most English language Bibles today. But, finally, I cannot see where the term canonical, in any sense, can be according them. How could there be two levels of canonicity? Of authority? What can be accorded them rightly is that stand as witnesses that there were faithful in Israel between the Testaments, that the true canonical Scriptures were regarded as the word of God, and that there was a real effort made in those intervening years to preserve and disseminate the Old Testament Scriptures. All of this provides invaluable background for understanding the world in which the ministry of the Christ was carried out.