Not just because she excites the Republican base,
But because she gives people further permission to vote for a white man over a black man.

Maybe I'm too cynical, but I believe that private racism is still alive and well in the US. Yes we've all but stamped it out in our institutions and laws (at least facially), but one cannot easily dissolve such an insidious practice that goes to the heart of the human condition.
Since the days of Cain and Abel, man has hated man--and will probably continue to do so until kingdom come. Whether over family, tribe, race, or religion, it is human nature to hate the "other."
Although we may "evolve" gradually, I don't believe that after decades of slavery, a mere fifty years of Civil Rights progress have radically changed the private hearts and minds of a significant number of white Americans.
So Palin gives people a free moral pass: I'm not racist, I voted for the white guy because I'm pro-woman!
But as politically brilliant a choice as Palin may be, I don't think it's enough for McCain to stop the Obama-train.
Anyone who gets more viewership than the American Idol Finale is pretty much unstoppable at this point!
(p.s. I don't by any means want to imply that people who vote for McCain are racist. In fact that's the beauty of choosing Palin, that's the whole point: It further obscures the race factor by putting the gender card in play.)