According to the help center, comments should NOT be used to answer questions. However I see many answers in comments. Other stackexchanges have a policy of deleting answers written in comments. We should have the same policy.
The logic is that there is no quality control. There are no downvotes to ensure that the answer is actually helpful.
Should we have a policy of deletion of answers in comments?
I think it's helpful to separate occurrences of this happening into two categories:
Sometimes the question being asked is based on a slight error or misunderstanding on the part of the OP, and all their problems can be solved with a quick comment like, "you dropped a negative there," or "you're close, but just change this to that." These comments don't seem appropriate as answers. Posts like this, after the OP gets help, don't really add quality to the site, so I would vote to close these as Off-Topic > Other > "It was a poor questing stemming from a simple error that was resolved in the comments.".
The other instances are of good questions that a user simply decided to answer in the comments. I think it would be nice to have a feature to flag the comment to be made into an answer, although I'm not sure how right it is to go against the wishes of the commentator that their comment be a only comment. But it would be such a waste to simply delete such comments. If they really answer the question, then they are helpful and add value to the site. My response so far to posts like this, if I have the time and motivation (which I seldom do), has been to create a community wiki answer that summarizes the answer that gets flushed out in the comments. That way the post can get off the Unanswered Questions Queue, and users can vote/comment on the answer as is intended on this site. The downside is that the commentator doesn't get the reputation for their answer, but I suppose if they are typing out answer-quality responses in the comments, they must not care too much. My CW answer reads something like this:
Note that userXXX has answered this question in the comments. I've posted (and possibly expanded on) their answer here, so that users of the site who confidently concur that this answers the question may upvote it.
Insert transcription/expansion/synopsis of the answers in the comments here.
While Comments are properly used to make clarification requests and tangential observations, not answers, and Answers should provide solutions justified by mathematical reasoning, I fear the proposed deletions of Comments are an invitation to "throw out the baby with the bathwater."
As I've previously stated, if someone feels I've given an answer in a Comment, the OP or another community member should feel authorized to post an Answer based on my Comment. This often happens when I've suggested an interpretation in a Comment that leads to a quick solution, based on the uncertainty that such a short treatment would really address the OP's problem. I justify such an exchange as a request for clarification, and I'm willing to "risk" that others will capitalize on such a request to provide a good Answer.
More crucially how would the proposed deletion of Comments be moderated? We cannot individually delete Comments, but must rely on flagging for Moderator attention. There would be no queue to review such flags, and thus it will ultimately require the Moderator to judge. Of course once an Answer is posted incorporating the information in the Comment, then a good argument can be made that the original Comment is obsolete/no longer relevant.
In general my feeling is that many Questions are in need of improvement before being suitable for retention on the site, and a certain amount of discussion in Comments has to be tolerated to bring them "up to snuff", e.g. adding suitable context.
I'm also sympathetic with Questions that provide context that comes agonizingly close to a solution. It seems to me that "hints" to close the gap in Comments are preferable to "hint" style Answers. Nevertheless the Community winds up tolerating many such Answers, and it should not (in my opinion) be a rationale for deleting (or forbidding) equivalent Comments.
Ultimately I prefer detailed Answers for all valid Questions because of the benefit to future Readers.
Disclaimer: I am among those with a fair share of what borders answers in comments, sometimes strong hints, though never a complete answer.
I believe that the asker of a question is often better served by pointing them in the right direction, rather than providing a full answer upfront. Down the road for them, that's the difference between remembering it as "I figured that out once, so I can do it again on my own now" vs. "I got the answer on MSE last time, let me take that shortcut again".
How MSE views/taxes such comments is, in the end, an inherently conflicting matter. On one hand, the site manifest says "MSE is a question and answer site for people studying math at any level and professionals in related fields", which would seem to imply that it's meant to help those very people who are asking the questions. On the other hand, it also says that "we're working together to build a library of detailed answers to every question about math" which moves the target away from the askers onto the future readers/google'rs. The two goals are not necessarily always aligned.
(For a crude example thereof, marking a PSQ as a duplicate is AFAICT welcome, even though that provides the most instant gratification to the asker. In contrast, answering a PSQ is a no-no.)
