Richard Bridge's definition of a purist is possibly defensible in some sense, though he is surely aware that judgments on form are made all the time. After all, that's what occasioned this thread. In any event, RB's definition isn't the one that comes to mind when most people encounter the "purist" bugaboo (tossed around prolifically and irritatingly, for example, in Sean Wilentz's Dylan in America; I would have thought that so eminent a historian would know better). It's also possible that when you get down to it, "purist" means not much at all; in discussions of folk-revival approaches, its true purpose may be simply to put somebody else down because his or her tastes in the presentation of the music are not exactly like our own.
In point of fact, of course, purity does not exist in the world. It's a construct like "authenticity." The past continues to influence us, but it is unrecoverable; we can't relive it, so we can only reimagine it and use it for our present purposes. Still, a Mike Seeger was closer to a kind of Platonic ("pure") ideal of folk music than, say, Bellowhead is. I love 'em both, and I am happily confident I am not alone.