However I see the point that we do need categories of some kind.
I get really annoyed when our local folk song society sponsors a concert and it turns out to be a singer/songwriter and has nothing to do with folk. In fact I think that is the one category we can all agree on. If a singer/songwriter sings only his* own original material then it isn't folk. One exception to this would possibly be a modern interpretation or version of a traditional song. This could marginally be considered a folk song.