I've always approached comments as transient things. Here one moment, gone the next. Great for quick clarification, but likely to disappear at any point in time. It's my understanding that the site based search doesn't work over comments.
If there is anything important to either the question or an answer it should be edited in.
In the interest of being civil I'll usually try and nudge the commenter to post an actual answer to the question. Then it can be correctly flagged as such to help future visitors to the question get to the answer faster. Then it gets all sorts of good things, like the ability to be edited and commented on in isolation.
That said, it's all to easy to miss an answer hiding in a comment thread. I've arrived at questions in the past, known the answer, answered as such and then moved on. If I find out there was a similar answer in the comment I'll try and indicate as such.
If people want to fire off a quick incomplete answer in a comment, that's all good. It will provide some immediate help to the asker. However, I don't think they should be surprised or offended if someone turns their comment into an actual answer (ideally with some attribution to them and fair warning that it will be done on their behalf).
To quote @sfdcfox's comment:
I use the site to help people, and questions with no official answers don't help people. I scour the unanswered list to see if I can provide answers anywhere, and comment-answered questions still show up in the unanswered list, which makes me look at them over and again. If the person who supplied the correct answer would make their answer an answer, or even if the OP would make a comment the answer, then I wouldn't have to, and I could focus on questions with no actual answers!