I'm not sure if it's just me, but I've been seeing a lot of answers that are being posted in comments. Nothing wrong with that, per se, but I would like to be able to upvote that as an answer that gives the commenter more rep.
Encouraging them to post it as an answer is an excellent way to remedy this.
Then there is Jeff's definitive answer:
I usually put an answer as a comment when the question is likely to be closed (or when I vote to close) but want to help the OP anyway.
I also put an answer as a comment when I feel my answer does not deserve the exposure of an answer but may help the OP anyway.
Finally, I use comment to put some humor sometimes that definitely doesn't deserve to be made as an answer.
Comments as answers are becoming more common on some sites. This is a problem because once a comment-based answer is posted, most users will not feel compelled to provide a proper answer. The question then remains squarely in the 'unanswered' column. Besides, comments-as-answers cannot be vetted, and voted on properly so the best answers don't always appear on the top.
The comment authors should be asked to post their comment as an answer. Failing that, comment-based answers are fair game. Post the answer yourself and flag the errant comment for removal.
I frequently use comments as answers if I think that the question doesn't meet our guidelines, but I feel like responding anyway. However, I usually mention that I don't think it meets the guidelines in my comment.
As I see it the answers in comments are usually poor quality and not of significant value. They detract from using comments to get to the real question. I have also see controversial answers posted in comments, possibly as you cannot be down voted and loose points for a comment. I feel that they should be discouraged, and ideally comment able to be up/down voted and points allocated.