In what ways does your vote count?
...I don't think my one vote can't change the course of a presidential election, I know it can't, to greater than 99.99+ percent certainty.
So why vote? For the same reason that some people (I admit I'm not one of them) agree to respond to Nielson TV-watching polls.
It's a poll. And for the president, your one vote can't possibly change who gets elected. (This is in contrast to local politics, in which a school bond in my parents town actually finished with a tie, with some 10,000+ votes cast.)
That's why it makes sense (if you're going to vote at all) to vote for the presidential candidate who is closest to your political views. If you are a libertarian, there is only one libertarian running in either party. That's Ron Paul. Mitt Romney is not even in the same league, stadium, or country as Ron Paul, as a libertarian. Ron Paul is a libertarian. Mitt Romney is not. And neither is any other other Democratic and Republican candidate.
The main thing you do with a vote is send a signal to political players about your views. The second thing you do with a vote is create a power bloc with which other candidates may have to negotiate and compromise. Very far down on the list of things your vote might be good for is changing who wins. Only for this last inconsequential reason to vote is the "they can't win" argument relevant. If you are not voting for the candidate you most agree with, regardless of their chances of winning, you truly are throwing away your vote.