Voting isn’t aggression, and it doesn’t constitute consent to be ruled. So a libertarian qua libertarian (and by libertarian I mean someone who accepts libertarian principles, not the broader Blockean meaning of libertarian) can have no objection to voting. Political philosophy has nothing to say about this matter. Which means the question ‘is it ever a good idea to vote?’ must be decided by something else rather than just political philosophy.
So we might rephrase the question by bringing in a specific goal: ‘is voting productive or counterproductive to the cause of a free society?’
I think there’s an error here. Voting is not the only thing that legitimizes rulers in the eyes of the masses. There are lots of things that legitimize rulers, such as the erroneous beliefs that rulers are necessary or beneficial for justice, equality, fairness, national defense, law and order, to provide a safety net, to steer the market, to protect workers from greedy capitalists, consumers from dangerous products, and on and on. All of these things make the state legitimate in the eyes of the masses too.
From this larger context, voting can be said to be productive to the cause of a free society, if voting is used as a means to help eradicate those all those other erroneous beliefs held by the masses.
You have analysed the act of voting in isolation, and yet you’ve used your conclusion to answer a wider question, a question which necessarily requires consideration of the bigger picture. The bigger picture is that there are many, many other reasons why people believe in states, and a Ron Paul presidency, for example, would do so much to eradicate these beliefs that voting for Ron Paul to try to make that happen far outweighs the benefits of non-participation, in terms of the effect on the perceived legitimacy of the state.
The night is young, the revolution is only just beginning, and the time for mass non-participation in the political and voting system has not yet come. There is more to be gained, at this stage, by playing the game and using it to our advantage, to reach a wider audience with our message.