Many good points were made in the course of this discussion. Yes, Obama's record of killing people is horrible. Yes, it's easy to reject Obama if you're a conservative who doesn't value social insurance anyways. Yes, it's a form of privilege to be able to not care about how much differentially worse Romney would be for people of color. But Henry Farrell's first post footnotes that it's abstracting away whether a single vote makes a difference in any case, and that's where the whole thing goes wrong.
Voting, outside those few and limited geographic areas in which your vote may actually matter, is primarily social signaling. No one doubts that just sleeping through election day because you don't care is being apathetic. But the people who don't vote and say that they're not voting are signaling something else, that they believe that the system has failed.
People all over the area where I live are talking about politics right now. I always have to drop in something like "I'm an anarchist, and I'm not voting because I think that people shouldn't support the American state." This makes me a jerk, especially when I say it to my daughter's third grade teacher who is teaching civics. But although it has almost no effect at all, it still has more actual effect than my going quietly to the voting booth and pulling the lever for Obama would have. Of course, I do some forms of political activism other than just talking to people as well, although they really have little more effect than this kind of thing does.
People on the left who don't vote and who say so are doing something. They are telling other people on the left that the system isn't working well enough. Not just the system of the corporations and the buffoonishly Randian right-wingers, who serve as easy targets for left-wing mockery. But the Democratic Party in America and whatever other supposedly left-of-center organizations still exist. The threat of exit may cause the people who run those things to eventually take notice.
And the question of what would happen if people on the left generally did as I did is nonsensical. If half of America believed what I do, the state would not exist in its current form. If just Democratic Party activists believed as I did, a candidate would have been chosen in the primary -- I don't know who, it would have been someone who is presently invisible -- who would not be blazing new trails in legal assassination. People don't believe as I do, and people in general don't believe as any of the people in this discussion do. The number of people who actually care about Obama's war and civil rights record is a tiny minority.
So we might as well act as a tiny minority, and not have discussions as if we were of any direct importance within electoral politics. Honesty is freeing. If I thought that my vote would actually affect the outcome, I would have to dutifully pull the lever for Obama, because yes Romney would be worse. But no one is going to be hurt whether we vote or don't vote, so any of us might as well loudly decline to if that's what they think is best.