If your vote is decisive, it will make a difference for 300 million people. If you think your preferred candidate could bring the equivalent of a $50 improvement in the quality of life to the average American--not an implausible hope, given the size of the Federal budget and the impact of decisions in foreign policy, health, the courts, and other areas--you're now buying a $1.5 billion lottery ticket. With this payoff, a 1 in 10 million chance of being decisive isn't bad odds.
Gelman also address the old voting-is-irrational argument, propagated this week by the Freakonomics authors once again. Here's Gelman's blog.
Here's a previous entry I wrote linking to articles by Gelman and others on the rationality of voting.