I agree with the previous two answers because:
1) Comments are not for low quality answers
Comments are for asking clarification, discussing an answer, etc. Even if the answer is a 1 liner, it should be an answer and not a comment in my opinion.
2) Helpful comments should not discourage complete answers
Some users might not have the time for a full answer, so they only write a comment. This should not discourage a user from offering a complete answer (if appropriate) even if 90% of it has been covered it comments.
There are a few other reasons too in my opinion, such as that it simply looks better to have an answer rather than a spiderweb of comments. Like the other posters said, don't plagiarize, give credit as a courtesy, it makes friends.
However I disagree with the other answers on the point of marking your answer community wiki. I think you should not mark it that way (unless you want to). First, if the commenters valued the reputation of the answer, then they would have written an answer, but they didn't, so you are not "taking" anything. Second, you are helping the site by putting answers in an answer location, where they belong, so you deserve credit for that.
I agree that you should "ping" the commenters to put their response into an answer first though. It is courtesy. But I doubt you'll get much of a response, and when you don't, it would be best (to me) to just complete the question like it should be completed.