I feel a little guilty here for having posted earlier and started the hare! The point I was drawing attention to was the fact that three of those mentioned already in the thread wrote their own material too, and thus were not strictly "traditional".
For most of the working performers I know it matters not where the song comes from as long as it fits. Martin Carthy was recently on TV singing Elvis! It's others who build the pigeon holes and try to stuff the performers in.