I never noticed this discrepancy before. We don't have folk-clubs in Ireland (or hardly any), but lots of sessions, and in singing sessions, IMHO, "performers" certainly make for good sessions. Fortunately we seem to be blessed with a lot of "performers" in Dublin, although many would disagree and prefer a more "artistic" approach to making music.

I think both can co-exist, but personally I feel sessions should be entertaining and fun, and that makes them successful.