"what the artists in folk clubs do with mick jaggers music turns it back into folk music."

I think this - along with Al's earlier comment about guitarists seeing things differently - is the crux of the question. And I used to agree (not with the bit about guitars, though). When I first got into singing at folk clubs, I thought this was precisely what was so good about them - you could bring along absolutely anything you liked, from the Stones to Dylan to Cyril Tawney to Richard Thompson to Peter Blegvad to your own stuff, and it all went into the same pot. (Somebody did "Angels" one night. Another night somebody did "La vie en rose". Come one, come all.) So that's one definition of 'folk' - perhaps not folk music or folk song, but certainly The Folk Experience. For a while I was really into it; I wrote a bunch of songs and everything.

Then I went to a traditional singaround and heard one amazing song after another, with stunning chorus singing from 15-20 complete amateurs all of whom (unlike me) knew every single song. And that gave me another definition of 'folk' - all the amazing traditional songs that The Folk Experience never lets you hear. I've been looking for that kind of folk ever since.

I'm not a purist - I sing whatever I want to sing, including new songs. But I found the motherlode six and a half years ago - in the second chorus of Ranzo - and I don't think I'll ever stop going back to it.