I personally voted to close some of my questions on other sites of the network. In particular I closed one recently as duplicate. I saw no value in deleting it because it might help the next person that would search for that kind of question and eventually find mine and not the duplicate (yes, it can happen, depending on the searchwords).
Also, deleting bad questions (or answers) is not something we should do indiscriminately, also bad posts have a value: they show new members what not to do here. If we continuously purge all "bad" (who determines if something is bad then? just a negative vote count?) content we might give the impression that this community is perfect, but we wouldn't be honest.