I'd rather the person ask me in the comments if I would move it to an answer than have the other person post it as an answer without asking.
Yes, people do that too, and I agree it's polite to do so. But if a comment just sits there for a long time, then under the present software, a commenter will not be alerted in mail to the fact that someone has made that request of him or her. Some things about MO are really inefficient!
There are times I purposefully leave things as comments to avoid rebumping the question
But if the question is never answered except in comments, then the MO bot will eventually come around anyway and bump it to the front page. (Hm, is that how it works? or does this happen only if no answer is ever accepted?) Anyway, I believe it's much more in line with the spirit of MO to give good answers (not just comments) to good questions.
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