Jim Harper provides an excellent response to the too-smart-by-half libertarians who pride themselves on not voting (and mock those who do). I'll add another benefit of voting Harper does not mention explicitly.
The usual anti-voting spiel goes like this. Your vote has zero chance of being the deciding vote. So what's the point? You're totally wasting your time. Not voting is smart. You should be smart. Like me. Harper responds by noting that the non-deciding vote also has value:
Votes are a dazzling roman candle of information supplied to elected officials, their staffs, political parties, journalists, opinion leaders, and future candidates, to name a few. All these witnesses to elections incorporate vote information—not just outcome, but win/loss margins—into their actions and assessments well beyond election and inauguration day.
Margins of victory matter: to candidates, donors, other officials, etc.
Yet voting has value apart from its direct effect on vote totals for various candidates or referenda. This is principally because many people see voting as an act of caring. If you vote, they think you care about your community/state/country. If you don't, they think you don't care and -- listen up, libertarians -- they will be less open to your ideas. Libertarians who want to influence other people might want to drag themselves to the polls if only so that they can later pass this test.
One might object that it makes no sense to use voting as a signal for caring. Perhaps, but it makes no less sense than using non-voting as a signal for smartness. We don't get to choose how others interpret voting. Sometimes, if you want to get anywhere with people, you obey the local customs, even if they seem silly.
I am not recommending that everyone always vote. There may be principled reasons not to vote. Many people who vote maybe shouldn't. But we should put to rest the "deciding vote" objection.
Your vote matters. Not as much as it would under instant-runoff voting, but it still matters.