Voting can be done as an act of altruism (in addition to other reasons), buying a small chance of very large impact, for which it can easily have a positive expected value.

I agree with this argument, but note that it only applies if you actually believe that the differences between the policies that Obama and Romney are likely to implement do amount to a very large overall utility differential (and that you can know beforehand in which direction it goes). I suspect that Konkvistador does not share this premise.