I wouldn't even call it purity testing, they're just testing. I've seen obsession over purity taken to a counterproductive extent, and I maintain that it can be a problem when dealing with a complex unideal reality, but what BadEmpanada is talking about here is fine. That's a healthy level of testing, and important in preventing recuperation or sanewashing. Democrats are a bourgeois-controlled party and don't share our class interests.

To give an example of the kind that is counterproductive, I know of a (small) socialist organisation in my country which has been banned from worker strikes after counterprotesting one, insisting that since industrial unions are bureaucratic, the workers should all just boycott the strike and make their own union. This group claims all other socialist organisations are impure and pseudo-leftist whenever they compromise with material reality and present conditions.

And, obviously, that's a whole other world of purity testing to what you're talking about. The problems are when it reaches no-true-Scotsman levels.