I liked Whistle Stop Steve's use of the term "the music to which I am drawn", and his defining criteria for this. Because when your involved in a session or similar, that's what matters, not abstract, though useful labels like "folk".
It occurs to me that this is rather like an imaginary discussion you might have about books in a public library. You're looking for books about, say ferrets (well, why not?) and you find them in various Dewey Classification Numbers, some geared towards natural history and some geared towards hobbies (and various others no doubt). I imagine you sometimes have arguments among librarians as to which was the right number for which book. And when it comes to keeping track of the books in the library,those classifications matter. But a ferret is a ferret all the time, with it's ferrettish ways.
As for whether the punk tradition belongs inside the folk umbrella, you could make a good case both ways. But when it comes to "the music to which I am drawn", Billy Bragg belongs there as much as Martin Carthy, and the Pogues as much as the Dubliners.
Oh yes, Whistle Stop - try out the forum searches, and if you've got the time you can catch up on the "what is folk" tradition of Mudcat discourse. A lot of good stuff in there.