This is the context where I originally heard the term 'purity' applied and this was well before the 3rd party explosion. Every now and then some guy would show up in a reissue discussion and arrogantly proclaim that he would never 'defile' his collection with cheap reproductions (referring to the official reissue). He didn't contribute anything to the discussion other than to turn his nose up at anyone who would collect a reissue.
After the 3rd party explosion, the 'purity' thing gradually shifted to 3rd party toys but often the intention of the poster remained the same.