You should not answer in comments. Not partial answers, not full answers. Not "leads on" an answer. Not "I would answer but I'm tired/just woke up/am drunk so I'll just say this..." Not answers that you think aren’t good enough to post as answers. Not little helpful tips, not helpful suggestions, not useful anecdotes. These will be deleted. Answer in answers.
And if your answer isn’t even good enough for you to want to put it in an answer post, just don’t post it at all then.
Answering in comments does the following things.
- It bypasses question closes. They're closed for a reason.
- It provides an answer that can't be marked as an answer for future people's knowledge.
- It contributes to long comment debates as you can comment on an answer, but it's unclear what you're commenting on in a comment thread.
- It is "cheating" by locking your answer to the top. Answers with higher votes/accepted answers should go to the top to indicate their quality. Bypassing that by sticking your answer in a comment on the question is unacceptable.
- It bypasses all our quality control mechanisms: we can't downvote your "answer", edit it, or comment on it to request clarification or improvements. Answers also bump a question to the top so that people will scrutinize the answer; comments don't do this.
- It gets in the way of people who are busy using comments correctly to improve the question.
The long and short of it is, every part of how how the site functions, all of which have lengthy justification as being part of the process of SE - rep, answers, accepts, edits, votes, etc. - is obviated by using comments for answers. So every good goal of all that functionality is nullified by this practice.
Now, "but the hapless questioner could use that info!" In nearly all cases someone posts the same information in a (much more comprehensive) answer. Or take the time yourself to write a real answer. We don't like crappy questions or crappy answers, and we'd rather not have the Q or A than to have one that doesn't meet site quality (hence closes/deletes, part of the standard SE functionality). If you don't care enough to write a real answer don't, the likelihood that you're the only person in the world/on the site that knows that bit of info is very small.
While users are welcome to steal the info in the comments to generate answers of their own, that will not slow the pace of dealing with the answers-in-comments via flagging and deletion.