I think he does more damage to the Republican brand the longer he runs. Yes, he will say mysogynistic, racist, homophobic shit. And yes, he will maintain a modest base of support. But the rest of the world will see him for the asshole he is, and he will drive sane people from the republican party.
I believe he's been hired by the RNC to run as a clown in order to make the other candidates look sane by contrast. If he absorbs all the hate-rays from the Left, that leaves little for Bush.
@6, The Donald is too ridiculous to hate. He's a dick, but the kind that I just roll my eyes at and ignore. I reserve my true ire for despicable assholes like Huckabee, or Santorum, or Rick Perry, or... damn, it's a long list.