That's one of the main problems I have with libertarianism (as it's often practiced in the U.S., at least). It's not pragmatic/realistic enough.
From this perspective, Conservatism is arguably the "best" ideology. Conservatives (in the political sense, not in the mother's-wombs-are-state-property variety, though they sometimes overlap) like to argue in political possibilities rather than theoretical possibilities. In this sense, I do tend to agree with political conservatism myself.
My problem with libertarianism is that it’s either inconsistent or too vaguely defined. On one hand you got those who beleive that taxes and governement is evil but are fine with having an army, police, justice system and public infrastructure paid by the state (the average minarchist). This is the inconsistent part. Either taxes are bad and need to be abolished or they aren’t and can be used to fund public services according to what democratic elected leaders deem necessary. After all, if governement is so bad, why would you let it control the army?
Well, I don't think Libertarians like Ron Paul oppose all taxes, just taxes they consider unnecessary (the Federal Income tax for example). It's hard to come by Libertarians who do not oppose state infrastructure but do oppose taxation. Unless Objectivists count as Libertarians, of course.
Then we have people who actually advocate a total abolishment of the state on the basis that it is evil. That’s consistent but I’ve never seen a clear explanation on how things would work in that world. Who would control police and justice? What would happen if we got invaded? How a privatly ownded system of roads would not end in chaos? "We’ll cross the bridge when we get there" is not an acceptable answer to me. If we are going down that road, we better have a clear picture of what is ahead.
Most anarchist libertarians are deontologists: If a state is evil to begin with, isn't simply the act of abolishing evil worth it? I find deontologism hopelessly naïve myself, but in order to debunk this form of anarcho-capitalism, it is necessary to debunk deontologism.
Also, more utilitarian minded anarcho-capitalists will say that the market already proved its merit elsewhere. In some ways, this is quite an attractive line of reasoning (like Communism, Libertarianism is very 'neat' ideology, in that can offer explanations for pretty much any political phenomena), but even if it were proven true, it would be a politically explosive thing to say the least, which is where we come back to the points raised by Quintillus.