It would be less aggravating if they weren't often wrong, or focusing on a single aspect or dimension of the original they want to keep pure, but forgetting about the rest.

For instance, some of those so-called purists have forgotten about the humour in JA's books. They think it's all about the Mother Of All Romances, so that if a story keeps the wit but not the pairing, it's impure, whereas a romance that takes itself too seriously (cf. MOAR) with the right pairing is acceptable to them.

I wonder how big the self-identification factor is for self-declared purists: all the MaryBennets who think they are Elizabeth.