Shimrod has it exactly right for me.
"A purist is a person who believes that musical boundaries and categories exist..... "
We put labels on tins so we know which one to open, nothing to do with whether the contents are 'good' or not, as long as they are what it says on the tin; that is a different criteria.
The problems arise when the term is used as one of abuse, that's when the crassness and acrimony starts.
"They usually perform traditional songs with a "traditional" voice, and the obligatory "traditional" Martin guitar."
As English traditional music was almost certainly unnaccompanied, no self-respecting 'purist' would dream of using a guitar.
It's that type of uninformed generalisation that gets up peoples' noses; I'm sure 'snigger-snogwriter' or 'talking horse' each gives just as much offence.
It seems to me that the majority of 'anti-purists' are the ones doing the condemning here and castigating those who don't like the same things they do - mind your own ******* business.
Jim Carroll