"If someone says to me, though, that such-and-such a song isn't 'Folk', I'll probably agree... if it doesn't fit the definition; it wont affect my relationship with the song, however, nor stop me singing it"

Fair enough, Brendy! No-one is stopping anyone from singing anything - nor would it be possible to do so or even feasible (or in any way reasonable)to attempt to do so. But I think that it is reasonable, and often necessary, to remind some people, now and again, what folk song actually is.