I’ll admit that reading Nozick (and following it up with Hayek, von Mises, Rothbard, etc.) turned me into a libertarian for a while. But the problem with Nozick’s view, as nearly every critic has pointed out, is that he doesn’t attempt to justify his intuition that people have Lockean-style natural rights. He just assumes it. Others, like Rothbard, have attempted to justify it, though I think without success. What Rothbard is, perhaps, more successful at is arguing that if you accept natural rights, then you are logically committed to anarchism, since, contra Nozick, no state can exist that doesn’t violate someone’s rights, so defined.
Now, I’m inclined to see Rothbard’s conclusion as a reductio ad absurdum of rights-based libertarianism. It just seems clear that the consequences of anarchism would be so terrible that there must be something wrong with the argument that gets you there. In this case, that would be the premise that there are libertarian-style natural rights.
If, instead of confining yourself to a natural rights position, you begin with a more consequentialist starting point, the justification of the state is pretty straightforward: everyone (or nearly everyone) would be a lot better off with a government than without it. Particularly, one might add, those who are weak or dependent in some way. Even supposing that the system of competing “protection agencies” beloved of anarcho-libertarian speculation could actually work, does anyone really want to live in a society in which your entitlement to not being killed, enslaved, or otherwise exploited was dependent upon your ability to pay?
This is where John Stuart Mill, incidentally, is better than some of his libertarian acolytes. Mill recognizes that the security provided by society is what later theorists would call a “positive” freedom rather than a sheerly “negative” one. Security of life, liberty, and property isn’t merely a matter of being “left alone,” but requires the state to take positive steps, devote resources, etc. And Mill is likewise clear that these basic rights ultimately have a consequentialist justification: a society that protects these basic rights is one that gives its citizens a better shot at flourishing.
The consequentialist justification of basic rights seems stronger to me than most natural-rights-style views, which often lean heavily on appeals to intuition. But once the distinction between positive and negative freedom is undermined, it’s hard to see why the government’s duties must be limited to those of a libertarian “night-watchman” state. All rights are positive rights in a sense, so why can’t rights to welfare, or health care, or what have you can, potentially, be justified on similar consequentialist grounds?