For the most part, closing a question neutralizes whatever harm it may have done. If you feel like the question is really worthless even for the community education purposes I described above, I guess you can vote to delete it. The only situation I can think of (right now) where you really should vote to delete is if there's a worthless question that has no chance of being reopened and somebody keeps editing the question to bring it to the top of the homepage.
]]>There are some closed questions which might be of interest to enough people, even if they are not appropriate for MO, that it would be worthwhile to keep them up. Others, like homework requests, I see no reason to keep.
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