Matthew Yglesias assures us we don't need missile defense, because "For starters, north [sic] Korea doesn't possess ICBM capabilities." That assumes a lot about the Taepongdong-2; this map shows how far those missiles could go under the broad range of estimates. And of course, there's always the possibility North Korea might continue working on its missile capabilities.
As for Russia and China, wouldn't the possibility that the U.S. might eventually have a significant defense give them an incentive to negotiate, on missiles and other security matters? I'd suggest asking Mikhail Gorbachev. And isn't there some value in protecting against an accidental or rogue launch, even if full protection against an all-out attack isn't attainable anytime soon?