The subjective and argumentative closure reason isn't meant to be used to close questions that can be viewed as subjective or can be argued about. Both of those criteria are necessary, in a "real" way (not just "I could imagine someone possibly arguing if they thought you meant this", but that the question is obviously likely to provoke an argument or be unanswerable), but they are not sufficient.
Questions should be closed as subjective and argumentative when they are bad subjective, as laid out in the blog post. In reference to the "well-regarded art-music composers" question, especially consider points 4 and 5 in the blog post:
4) Great subjective questions invite sharing experiences over opinions.
You can't really have "an experience with" regarding composers, that doesn't really make sense. But regarding/evaluating someone as "innovative" is definitely about opinion. Solely about opinion, in fact. What music people like is a notoriously subjective topic, and strong disagreement is guaranteed.
5) Great subjective questions insist that opinion be backed up with facts and references.
You can theoretically find other people that think certain composers are innovative. That's not a hard fact, however -- the fact is that other people have opinions, not that the composers are innovative. We already know people got opinions :P
As well, as I said in a comment on one of the answers to the meta question:
I highly doubt the contemporary art composition community (which is a subjectively defined group) has a fixed list of their opinions that all members agree on. Any answer would be too localized, as well (new composers come along, some die or retire or stop innovating, whatever; the list would quickly be out of date).
A question should also not be evaluated according to its answers. While it's wonderful that we are receiving great answers on this site, bad subjective questions solicit bad answers. That should not be encouraged, even if it's not a problem at this very early stage of the site. You are right, though, that we should also encourage good answers that are not provocative or over-argumentative.