There are two types of political scientists: those who think there is a duty to vote, and those who think voting is irrational. I'm in the latter camp. The argument for a duty to vote was elegantly stated by H.B. Mayo.
The usual argument that voting is a duty of all eligible to vote runs something like this. A successful democracy depends upon widespread interest and participation in politics, among which voting is an essential part. Deliberately to refrain from taking such an interest, and from voting, is a kind of implied anarchy; it is to refuse one's political responsibility while enjoying the benefits of political society. A right of non-voting, if widely exercised, would hasten the end of a democratic government; non-voting is a mode of action impossible to universalize in a democracy, and so fails to meet Kant's test for the categorical imperative. (citation below)But that's a very normative argument, and it fails to refute the positive argument. The critical component of the positive argument is the undeniable fact that my vote won't make a difference in the election--that is, the expected utility of voting, if the value sought is "effect on outcome," or "getting the policies I want," is precisely zero (in a presidential, or any other large turnouot election); or, put another way, voting has little to no instrumental value. (There is a vast literature on this, which is all footnotes to Anthony Downs.)
Some people find that disturbing--I find it freeing. Some people worry that a vote for a third-party is a wasted vote, but I tell my students that since your vote can't change the election, it's real value to the individual voter is it's expressive value--that is, voting can have "intrinsic consumption" value. Therefore, you waste your vote whenever you vote for someone who is not your top choice, because you have reduced it's real value to yourself. (Of course that doesn't hold true in cases where your vote can make a difference--then strategic voting, casting a vote for your less preferred alternative, to block your least preferred alternative from winning, is entirely rational. And if your expressive value comes just from the act of participating, of supporting democracy, then you could vote randomly and still capture the intrinsic consumption value.)
That's why I regularly vote Libertarian. Sure, the Libertarian candidates are invariably nutcases, but I express my preference for less government regulation that way. (In my dreams, enough people vote Libertarian that one of the major parties takes notice and co-opts us by shifting away from their love of government regulation to a moderate libertarianism).
And that's why I might vote for Obama (assuming he's the candidate). In fact, I think we have much better choices for president this year than we have for many years. McCain has far more foreign policy understanding than Bush, Kerry, Gore, B. Clinton, Michael Dukakis, Reagan, or Carter did when they ran for office. H. Clinton knows what the presidency is really like, from her eight years in the White House, and wisely chose to serve on the Senate Foreign Relations and Armed Services committe, to gain crucial knowledge about those issues, so vital to the presidency. Obama lacks that experience, but appears to have real integrity, not just a political pretense of it. I think he's a man of honor, and after Bush, Jr., and Clinton, I'm ready for that. Better a man who honorably makes mistakes, than one who dishonorably does. And I hope, reasonably I think, that he's smart enouogh to surround himself with experienced people.
I honestly could be comfortable casting a vote for any one of these people, so the question is, which would provide the greatest expressive value for my vote? Foreign policy-wise, John McCain. But breaking the gender and color barriers are also important to me, and on election day I might vote for the Democratic candidate just to express that value. If my vote could tip the election, I'd vote for McCain. But my preference for him is not nearly strong enough to automatically outweigh these other issues.
Mayo, H.B. 1959. "A Note on the Alleged Duty to Vote." Journal of Politics 21(1): 319-323.