I think it's fascinating how English rock music and skiffle created the--what was it, the second wave? of the English folk revival. I mean John Renbourn, Bert Jansch, and Martin Carthy, if I'm not mistaken, all were very influenced by skiffle and American blues players like Big Bill Broonzy and Blind Boy Fuller. Besides that, it seems to me that Eric Clapton single-handedly created interest in Robert Johnson.

Of course, since the 50s and 60s, rock and folk music have diverged further, but they do come from similar roots. Folk musicians--even the "greats"--don't exist in a vacuum, and I think a truly great musician can learn from any type of music he or she is exposed to.