I have seen some folks suggest that Magisterial infallibility is unnecessary, because we do not need whatever certainty that this charism affords us. The upshot, apparently, is that it’s okay to have a measure of uncertainty associated with the Christian faith – or at least with some aspects of it.
It’s one thing to say that we don’t need what I’ll call “infallible certainty” (although that is a somewhat misleading label, as though my personal certainty is what’s in view!) for literally everything related to the Faith, but it’s another thing entirely to decide what stuff we really should have infallible certainty about if not everything. And of course the question necessarily comes up: who decides what we need to be certain about and what is less necessary or less certain?
If we say that literally nothing associated with the Christian faith requires infallible certainty, or that we cannot be certain about literally anything associated with the Faith, then it seems to me that we might as well all pack up our things and go home after paying a visit to our non-Christian friends to apologize for our exclusivist claims. On the other hand, once we say that there are some things that we may and must hold with certainty, it becomes necessary to be able to identify them. And as I said in the last paragraph, that means that someone is going to have to decide.
Will it be an every-man-for-himself arrangement? If so, it seems that there is no principled difference between this and no certainty at all: if every man decides for himself what things we can and must have certainty about, we have no warrant for criticizing other Christians’ beliefs nor even those of non-Christians.
Will it be theologians that decide? If so, which ones? Who decides which theologians will decide the question? And if someone is selecting them, is it not the case that we would be submitting to the authority of The Selector(s) when it comes to establishing what we need to be certain about? And why should we suppose that scholars are even equipped at all to tell us what we must be certain about?
It seems to me that the only reasonable course is to rely upon the Church to do this, precisely because it was established by God Himself. Any other course is arbitrary.
Aquinas, Etc. 
Added links to the Aquinas articles at your suggestion. Thanks for reading.
My pleasure, Drake.