Does it fully and completely answer the question? Post it as an answer.
Does it not fully and completely answer the question? Don't post it as an answer.
The crux of this issue seems to be less about whether or not to post an answer as an answer, so much as whether or not to post an answer as an answer even if it's not a good answer.
If it's a truly bad answer, it probably should not be posted anywhere. But if it must get posted, it's important that it gets posted as an answer so that the community peer-review can actually judge its merit and, if necessary, vote it down so that everyone else can see that it's a bad answer.
If it's an okay answer (but not a great answer) it's still important to post it as an answer. The community peer-review is essential to how the Stack Exchange model works; if only great answers are posted as answers, while "okay" answers and "bad" answers are relegated to comments, there is absolutely no way for future users to rely on peer-review to determine which of those comment-answers are "okay" and which ones are "bad" (especially if there's still no "great" answers to read instead). And with no way to distinguish "okay" from "bad", it's pretty much impossible (or at least highly disrecommended) to trust any comment-answer for any reason.
And if the answer can't be trusted, there's pretty much no value in posting it.
I have seen many users use comments to post answers explicitly because they're trying to avoid downvotes. For users who understand how the Stack Exchange model works, this is usually a very good sign that the poster doesn't actually trust his own answer, or knows that it's not even welcome on the site as an answer for whatever reason. In other words, they know that any such answer needs to be taken with a grain of salt.
For future users who don't know the Stack Exchange model, however, this will often be the first "answer" they see; comments can garner upvotes which promotes them to the user's view, but they by design can never be downvoted, never be improved, and are pretty much immune to any form of constructive criticism or peer-review that makes Stack Exchange answers actually valuable. This gives them undue weight which makes them seem more valuable than they actually are.
This is why I'll purge answers whenever they're posted as comments; whether they're "right" or "wrong", "good" or "bad" is irrelevant: Comments have a very particular use, and that very particular use is not answering questions.
If your comment provides a hint or a suggestion that can help the questioner or a future answerer to develop a full answer, go ahead: Comments are great for that. If you're not 100% sure about your answer, go ahead and say that you think maybe possibly the answer might be such-and-such but you're not sure and will look into it later, knock yourself out: Speculation and guesswork isn't an answer, but it can totally help others find the answer.
But comments that deign to answer the question itself, even if you don't have the time or inclination to make it great instead of just okay, should still be posted as answers. Or not posted at all.
(As a side note, any question that can be fully and completely answered in a single comment is possibly a terrible question in the first place.)
Even if you can't be bothered to dig up all the references and do all the research to make an incredible answer, that doesn't mean your answer is worthless, or even bad; sometimes any answer is still better than no answer. But if nobody's posting okay answers as answers, nobody else is going to feel encouraged to do so either. So you end up with a bunch of unanswered questions, or questions which attract a lot of bad answers, not because because nobody knows the right answers but because nobody is willing to post the right answers because they're not "great" enough.
We're trying to make the Internet a better place here. As long as posting an "okay" answer is better than the alternative, post it. Otherwise, don't. But either way, an answer is still an answer, not a comment.