Yes, usually the answer is right there on the first page and it says "Closed as not relevant" (but fortunately it's usually been answered with a helpful answer anyway).
That's a good point. I never really got the 'not relevant' bit. After all, if a question was worth asking it was worth answering (there is no such thing as a stupid question or an irrelevant one, there are stupid answers though. The closest thing to an irrelevant question (or a stupid one) is a question that wasn't asked...).
SO would not suffer unduly from letting these not relevant questions be asked and answered anyway. Relevant Questions are like beauty, they are in the eye of the beholder.
>there is no such thing as a stupid question or an irrelevant one
I think that's where they disagree with you. Stack Overflow aims to keep discussions as minimal as possible and is not the place to weigh the pros and cons of programming language X against that of programming language Y. If you want more information, they cover their take on the issue in great depth on their podcasts (which are great to listen to, regardless).
I like what they've done. By closing all bad questions and a few good questions the site contains only good questions. The obvious downside is that the occasional interesting question is removed, but I think the trade-offs are worth it. And judging by the popularity of the site, I think most everyone else does too, even if they're not consciously aware of it.
SO would suffer, because the kind of question you're talking about is intrinsically debatable. Those questions aren't bad or wrong, but they are inappropriate for the medium of SO, which is about technical questions with concrete answers. SO is not the be-all, end-all of knowledge, but it is pretty close to perfect for technical questions with right answers, and I think it would suffer greatly if it developed an HN or reddit-like clubhouse atmosphere.
If it isn't obvious to you I doubt anybody will be able to "prove" it to your satisfaction. Besides, you can't create rules around impossible conditions like "close any question which will lead to endless debate" so instead you create rules around easily detected conditions like "close any question that isn't concrete."
I don't greatly love SO (though I like it more than I used to) but there are lots of alternatives that lack SO's irksome limitations (slant.co comes to mind). By now it is obvious that whether you like SO's formula or not, it clearly works.
A "not relevant" question is usually not a "bad" question, but just a question that can't have a good, definitive answer. SO are trying to be a site hosting such answers, not discussions.
So opinion-based questions are explicitly unwelcome on SO. They can collect lots of worthy information in comments, so such questions are usually closed but not deleted.
SO even tried to channel the discussions by developing discourse.org so that interested parties can get a them a discussion forum and continue there :)
It was Jeff Atwood (and friends), not SE, that developed discourse. One of the reasons he headed in that direction was because of the discussions that weren't appropriate for SE sites - but it wasn't SE trying to fill the discussion need.
SO would not suffer unduly from letting these not relevant questions be asked and answered anyway. Relevant Questions are like beauty, they are in the eye of the beholder.