Jon Stewart has proven over and over again, in multiple ways, that he has a combination of traits that make him the ideal debate moderator. Stewart is one of the few public figures who possesses an unshakeable grasp of the facts, an agile and fearless mind, and an ability to speak truth to power, all with a sense of humor that makes it all entertaining. If Stewart were to moderate a presidential debate, viewership would in all likelihood be the highest it has ever been (which would empower more citizens to vote in an informed way). If Stewart were to moderate a presidential debate, candidates' attempts at shenanigans would be stopped dead in their tracks. (There's no way Bush could've gotten away with his "LET ME FINISH! LET ME FINISH!" B.S. if Stewart had been moderating). If Stewart were to moderate a presidential debate, we know that the tough questions would be asked, that no stone would be left unturned, and that we'd all come away from it with smiles on our faces, a much deeper understanding of the candidates, and a sense that maybe everything's going to be okay after all.
You think he's just a left-wing shill? Fine! Let's have Bill O'Reilly co-moderate (y'know, to make it "fair and balanced" and all that.