Let's put it this way, the two reasons you've given for not voting are:

1) You're unlikely to affect the outcome anyway.

2) If enough people don't vote the government will have less legitimacy and this can have positive effects.

Since the logic of these two reasons contradict, would you mind telling me which is your true rejection?

I'm another non-voter, largely (or medium-largely) for the reasons Konkvistador gives. But it's not the legitimacy of government that I wish to weaken. Places where government, even bad government, is not taken seriously are not nice places to live. If there's an institution or a cultural value that I wish to see weakened it's the people's romance.

In general I see nothing inconsistent about a democracy where most people voluntarily abstain from voting. A norm of not voting would require low amounts of sectarian conflict and large amounts of social trust, which don't exist in very many democracies. But as goals go I think low levels of sectarianism and high levels of social trust are superior to (and at cross-purposes with) high levels of voting.

We still want the state to have enough legitimacy to secure property rights and enforce contracts.

You are right. I concede it probably isn't instrumentally useful for the goal of a small, strong and stable government capable of enforcing contracts and protecting rights. While the de-legitimized state might have a hard time growing even more and in its incompetence new de facto freedoms would slip out of its fingers, but the freedom is the freedom of anarchy not the liberty of minarchy. The argument I gave degenerates into a basic argument for anarchy and revolution in the hopes for change. Something that has historically almost never worked out well.

Since the logic of these two reasons contradict, would you mind telling me which is your true rejection

Good catch. I don't think people not voting has a large effect, just that people not voting also sends a signal to the system and it doesn't seem obvious that it is much smaller one than the one you send by voting for a party or candidate.

1) You're unlikely to affect the outcome anyway.

2) The tiny expected influence you have on the outcome doesn't go away when you don't vote, because abstaining from voting is also a political act.

I would perhaps add 3) that this political act may have instrumental utility for certain kinds of goals.

But applying 1) and 2) I get a bit of a problem. My value of information argument against spending time on thinking about party politics should then also clearly apply to thinking about voting or non-voting as well, advice I'm obviously not following. My revealed preferences point that some part of me thinks that not voting is very desirable. This can't be argued on consequentalist grounds for the reason you point out. Thinking about it I seem to consider non-voting valuable enough to think and talk about for symbolic reasons, seeing it as a sort of Schelling fence of personal political detachment from one's society. If you live in a society where your values or map of the world radically diverge from the rest of society, such a thing is perhaps good for personal well being, seeing oneself as a subject rather than a citizen helps you deal with the constant pain of things going horribly wrong.

Looking from the outside I'm using non-voting arguments to try and promote alienation from the society and hopefully drift towards my mind space. My inside feeling to the contrary is weaker evidence. Readers should then try to correct for this.

Taking another step up the ladder, perhaps my self-proclaimed divergent values are only a rationalization for my lack of tribal feeling linked to the state. Such a predisposition is hardly unique in the mindspace near LW/OB.

Why put so much distance between myself and the outside world? Because despite my legendary optimism, I find my society unacceptable. It is dreary, insipid, ugly, boring, wrong, and wicked. Trying to reform it is largely futile; as the Smiths tell us, "The world won't listen." Instead, I pursue the strategy that actually works: Making my small corner of the world beautiful in my eyes. If you ever meet my children or see my office, you'll know what I mean.

I'm hardly autarchic. I import almost everything I consume from the outside world. Indeed, I frequently leave the security of my Bubble to walk the earth. But I do so as a tourist. Like a truffle pig, I hunt for the best that "my" society has to offer. I partake. Then I go back to my Bubble and tell myself, "America's a nice place to visit, but you wouldn't want to live there."

My politics and values are quite different from Bryan Caplan's, yet the conclusions seem remarkably similar. Maybe both of us already had our bottom line written out first.