Occasionally we would get questions back. There is a point at which you have to stop answering questions, though. We're not a tutoring service. At that point we'd just say something like, "We don't have any more detailed feedback other than what we've given you. Good luck on your job search" kind of response. We never got beligerent replies from candidates, but sometimes got them from recruiters.
The recruiters are the real problem because they will sometimes (some of them even often) demand "partial payment" for candidates that they thought were "good enough" but who we rejected. We had to write a few strongly worded "Our decision is final and we believe it is justified" letters to those recruiters. Saying that we won't accept any more candidates from the recruiters if they persist shut up most of them, but not all of them.
Eventually we gave up using recruiters anyway because they were essentially giving us random candidates. I think if you can find good recruiters and you can build up a good rapport with them, this kind of feedback is as useful to them as it is to the candidate. If you can't, then you are better off without them.
So, if anyone is wondering if it's worth doing, I would say that my experience is generally positive.