Ironically, I have a lot of time for purists as in taking a hobby seriously and combining the historical research of how we expressed ourselves and getting good abstract enjoyment out of perpetuating that medium.
Lots of big words there.
Nonetheless I see the difference between a person taking a huge interest in what we loosely call folk and somebody berating others for not fitting their ideal.
Just to keep the purist term a neutral one, I don't think a bloke who goes in to a barmaid and asks for her to Allow him to partake of a jar of her finest real ale is a purist, he is a prat who cannot ask for a pint of bitter. I see similar prats once they have their pint and are discussing the merits of a song some poor sod is trying to sing. They are not purists, they are prats. a purist may know a bit about the song, might even understand what specific gravity means with regard to the keg beer disguised with a clever hand pull valve, but purist they are not.
A purist for me is somebody who then can't understand why we don't all embarrass the bar staff and piss off the singers.