The community actively deletes enough poor quality questions to keep the site reasonably clean.
Questions are only eligible for immediate deletion by the community when they score -3. In my experience, the number of those questions that might be useful to someone else is vanishingly small. They are the usual vague, underspecified, shoddy questions that we've all come to know and love.
The remaining questions may or may not get deleted (after a 24 hour wait), depending on whether or not the community wants to keep them around. Many of them may stay on the site for quite awhile due to lack of interest, which is why we have some automated deletion algorithms now.
Of the rest (zero or positive score), I'd say that relatively few that are useful to others actually get deleted. The ones that are... well, they're just not useful to anyone else. It's extremely rare that a question gets deleted if it might be useful to someone, even if it's closed.
The ones that are useful but do get deleted are contentious in some way; they've managed to skirt the community rules for question quality, gather a lot of upvotes, and get closed and reopened. Community deletions are a pretty effective (but difficult) way of disposing of them. I say difficult, because it takes many delete votes to delete these kinds of questions.
You can get a very good feel for the kind and quantity of questions that get deleted (and undeleted) on a regular basis by looking here (10k only).