I reread the tools menu more closely and learned that people with 10k+ reputation have the power to vote to delete questions that have been closed for two days or more. Does anyone think this is a power worth exercising?

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.

My reasoning for keeping closed questions around is to make it easy for people to learn what kinds of questions get closed and what kinds of reasoning goes into it (in the comments). This serves a couple of functions. One is that we can direct people to examples of closed questions when their question gets closed, so they'll know that we're enforcing a consistent policy, not just picking on them. Another is that regular users get to see how and why questions get closed, so when they get to 3000 rep and they can vote to close, they have some idea what they're doing.

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.