Yes, people do use the Mythbusters card in discussions, but what's wrong with that? Mythbusters tested the myth, there was an outcome, and the discussion is solved. Whoever didn't use the Mythbuster card lost.
If you're annoyed by the people who always use the Mythbusters card, even if the Mythbusters never even tested that particular myth, then I agree. That's stupid.
I also agree that sometimes the show gets it wrong and tests the myth using inaccurate circumstances, but that's a rare case. I'd say 95% of the time they test the myth legitimately, bust or confirm it, then ramp it up to massive exploding levels anyway.