"Does it really matter if all of D - X above are sung alongside traditional songs as long as the traditional songs are being sung,"
Yes it does - it robs it of its uniqueness, especially as there are those, here and elsewhere, who, for one reason or another, would argue that the tradition is no more than a collector's/researcher's wet-dream.
I played with interest a clip put up by somebody here of a selection of what goes on in their club. I was knocked out by one of the examples, but found that, by the end of my listening them all, I needed to replay it as it had merged into the 'imo' mundaneness of some of the others - a luxury you are not offered in a folk club.
Whether we like it or not, no matter how wide their tastes, everybody compartmentalises their music - try singing a traditional ballad, or performing a piece of chamber music at a pop venue (which, it appears to me, many folk clubs have metamorphosed into) - and then get ready to run for the door.
I think Pip just summed it up perfectly.
Jim Carroll