Moore has good instincts and picks good fights, but he tends to play fast and loose. At his best he brings attention to topics that are getting ignored and gives a prominent voice to people who are getting excluded, and at his worst he's being either sloppy or dishonest and badgering underpaid security guards who shouldn't have to put up with that shit.
I don't agree that he hurts his causes, though. It's not like Glenn Beck damaged the GOP in the midterms, or that Al Gore's deliberately nerdy, science-oriented approach to policy didn't contribute to a huge crazy backlash against climate science. Moore's a controversy artist that fires some people up, turns other people off, and probably doesn't make a huge impact either way (though I think GM was forced to keep a couple plants open because of Roger & Me).
PS I really like TV Nation back in the day, but I also thought Bill Maher was great back then, so who knows.