Andrew Gelman defends the most basic one:
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.
The Dish 