Reid attacking other progressives doesn't happen much on the show, and tends to be largely constructive in a sense. She's mainly arguing about strategy (she seems to favor boosting minority turnout vs reaching out to reluctant Trump voters), which isn't exactly taking people's heads off. She does snatch up bullshit conservative talking points though, and is able to do them in relatively short soundbites. Those are very valuable to our side of the culture war, and they often seem to go viral, which is a far more effective means of distributing ideas than simply live broadcasting.
I think alot of the ideas around her "credibility" and such are founded in the old media environment where you pretty much had to get your ideas out live, and live audiences were the most important. That's clearly changing now, as reflected in the meme war the Russians fought during the 2016 election. You can also see it in the changing economics of the media landscape- recorded media tends to have a long tail, and will soon become far more economically valuable than recorded content. TYT has had major success in that area (see Hasan "the Hun" Piker) and virtually all of the broadcast news companies have abandoned their own online video platforms for Facebook and YouTube.
I say all that to say, in that environment credibility doesn't matter that much. Or at least as much as it used to. What matters is whether you can get your point across in an entertaining, yet informative fashion in 3 minutes or less. Joy is good at that. Especially when it comes to destroying Republican talking points and promoting stuff like Moral Mondays. Mellisa Harris-Perry was better at it, but MSNBC's dumbass execs ran her off.
