Stephan Kinsella takes an unusual, "pessimistic anarchist" position. Since he has just recently set it out at length, I will take a moment to point out what I think to be some problems with his views.
"Accordingly, anyone who is not an anarchist must maintain either: (a) aggression is justified; or (b) states (in particular, minimal states) do not necessarily employ aggression.
"Proposition (b) is plainly false. States always tax their citizens, which is a form of aggression. They always outlaw competing defense agencies, which also amounts to aggression."
So that handles (b), does it? Of course, someone who believes the state is legitimate doesn't believe taxation to be aggression. Naturally, if you get to define the terms the way you want, you can win any argument, but really it's an empty victory just to define your way to the win.
"Conservative and minarchist-libertarian criticism of anarchy on the grounds that it won’t 'work' or is not 'practical' is just confused... Consider an analogy. Conservatives and libertarians all agree that private crime (murder, robbery, rape) is unjustified, and "should" not occur. Yet no matter how good most men become, there will always be at least some small element who will resort to crime. Crime will always be with us. Yet we still condemn crime and work to reduce it."
Well, it's an analogy, but a bad one. The people who claim that anarchy won't work are not claiming that the State is a negative factor in society, like crime, with which we must learn to live. Instead, they are claiming it is necessary for social order, and that eliminating it, while not impossible, would be disastrous. And, if this argument is correct, then the State is morally defensible, as necessary to human social life, and its taxes and suppression of other defense agencies would certainly not be forms of aggression.