It's not a workable technology. And now they are going to test it a grand total of three more times before they decide whether to deploy it or not. It's buggier than Microsoft software. And even if the technology were miraculously to work during tests, it would be easy for an enemy to elude or confound the missile defense system once it was set up. The history of the arms race is littered with innovations that were supposed to secure one nation's safety or superiority but proved ephemeral at best and destabilizing at worst.
Missile defense is just the latest.
And the irony is, it doesn't even address the most likely threat against the United States. That threat is not from North Korea lobbing a nuclear missile at us. Such an action would be suicidal. Kim Jong Il is not stupid. He didn't wait this long to inherit the family business only to throw it all away.