When a rogue state like North Korea has the capacity to launch a cloud of atomic missiles at us, by that point missile defense may be the least of our problems.

North Korea will, even if it continues development, probably never have more than a few nuclear-tipped missiles capable of reaching the U.S. If they sell one to some psycho, a not-unlikely scenario, then he'll have one.

A system effective against even one or two conventional missiles will already be worth the money spent on it. And you can keep improving from that.

Missile defense saves lives. Not only American ones, but lives in the attacking nation. A rogue missile shot down over the Pacific is going to generate a different and probably less-severe military response than a missile that hits Seattle.