I have literally never seen that argument, and frequently seen the argument that communism (and/or socialism) is Bad, because the USSR was communist, and they were Bad.

Not that I'm saying you haven't encountered the reverse; I'm quite willing to believe that people who run in other circles (or make other claims) encounter different arguments. But yeah, I see the "we shouldn't want communism/socialism, it killed millions of people under Stalin" argument all the damn time.