For the Stackexchange system to work properly, we need answers to be posted as such. Lately, I have seen a lot of comments that contain at least a rough seed for an answer posted on questions.
While it is nice to have an encompassing answer, short answers are helpful, too! You or another user can always come back and improve your answer at a later time, if you think of more to say, or get requests for clarification.
Answers in comments discourage people from giving the same information as a full answer, but miss a lot of the benefits of answers! Comments clutter up the reading space, they don't reorder for votes, you cannot suggest edits on them, and voting is restricted to upvotes and flags.
Comments are supposed to be temporary notices to the author of the post, either for requests of clarification, constructive criticism or minor information such as a relevant link.
Check out privileges/comment for a full overview on comments (summarized below):
When should I comment
You should submit a comment if you want to:
- Request clarification from the author;
- Leave constructive criticism that guides the author in improving the post;
- Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the question has been updated).
Comments are not recommended for:
Answering a question (rather post an actual answer), Suggesting minor corrections, Compliments, Criticism without feedback, secondary discussion, discussion of site policies