What does the Palin pick do for John McCain? Three things, all of them political, as far as I can tell:
1. It reinforces McCain's image as a maverick.
This caught almost everyone by surprise, particularly the media. Just when it looked like McCain would have had someone like Romney forced on him by Party insiders, he did something very unconventional.
2. It rallies the Republican evangelical base.
A mother of five with the oldest in Iraq and the youngest born rather than aborted despite Down syndrome is inspiring for all but the poster child for the Religious Right. Just when all of the pundits thought this was going to be about appealing to the moderates or making peace with a former rival, McCain goes and shores up the Party's base. Some southern states that might have been within Obama's reach now will require more intensive campaigning.
3. It divides his political opponents.
I don't think that McCain will gain the vote of many women who would have voted for Clinton but not Obama because he picked a woman as his running mate. But, like the Clarence Thomas nomination to the Supreme Court, putting Palin on the ticket requires those on the Left who play identity politics into a less comfortable position by having to attack one of their own. Take a look at NOW's press release for an example. (Still no word on Golda Meir from NOW. Perhaps she didn't support women's rights either.)
That's quite a political trifecta. What is the cost of McCain's political gamesmanship? If it turns out that Palin really does have the mastery of national and international policy issues that a vice president should, then there is no cost at all and her nomination is a very good thing for the country. If she is found lacking in competence, then the cost is far too high to contemplate. I'll be looking forward to seeing her on the campaign and under scrutiny to decide.