In my experience (and IMHO), a "purist" is simply someone who has adopted one particular viewpoint as their own to the (unfortunate) exclusion of others.

To be a "purist" is very, very difficult as far as I can tell. It seems as though everyone eventually has an experience in which their uniquely 'correct' or exclusivist position is shaken by the advent of another valid position.

Take religion, any religion, for instance. Claims of exclusivity are necessary, I suppose, on some level if the religion is going to claim to be relevant on its own merits. To go so far as to say that "only the TRUE BELIEVERS in OUR religion, who possess the (spirit, secret, code, handshake, apron, whatever), are able to attain to righteousness..." is not only hard to prove, but it's usually laughably easy to DISprove. All one has to do is locate someone who is kind, loving, or manifests whatever character the "true believers" have, only without being a "believer" themselves.

See? A folk purist, like any other purist, is only making a valid point (again, in my opinion) when they are preserving and cherishing a tradition, not insisting that their tradition is totally pure and sacrosanct and untainted and must be reverenced as such.

Except for MY tradition... mine's the pure one. ;)