Questions should be deleted when their content no longer adds anything to the site. Questions are closed for a variety of reasons, so let's look at each close reason and whether they should likely be deleted:
It depends; Look at the context of how they are asked.
You'll want to keep the question if the wording provides another way for a search query to find the content. It's a bit like a "see also …" entry in an index.
Delete it if the duplicate does not add terminology or alternate phrasing to find the question. It may not be worth cluttering up the system with this exact duplicate. There are several poorly phrased questions that will not direct traffic to us.
Almost always delete it.
Off topic usually says "This shouldn't have been posted here in the first place." So, for anything not about topic [x] or the off-topic reasons, just let it burn!
Delete it if it says "this is nothing but a 'broken window', and we should get rid of it." This includes lists of questions where the only information is about 2 lines of text per answer or Wikipedia/internet dumps.
Broken window theory: The theory is that minor forms of bad behavior encourage worse ones: that a neighborhood with lots of graffiti and broken windows becomes one where robberies occur.
Always delete it.
Closing it before deletion provided ample time for the author (or community) to fix it up. If it wasn't re-opened by this time, it should probably go.
Primarily opinion based:
See "Too broad".