North Korea clearly intends any launch as an act of intimidation, part of its long-held belief that nuclear threats give it political leverage. Knocking the missile out of the sky, or even trying to, would tell the North that it can't succeed with such tactics. It would also reassure Japan and other U.S. allies that we have the will to protect them from rogue madmen. The demonstration effect would be useful around the world, not least in Iran.It sounds good to me. James Joyner and Blue Crab Boulevard anticipate problems.
As North Korea weighs a launch, it's a useful moment to recall how we got to this pass: Amid the arms-control era of the Cold War, the U.S. chose to defend itself against attack by plane or ship or ground but not by missile. One reason North Korea--and Iran--decided to invest scarce resources into developing nuclear weapons and ballistic-missiles is simply this: The U.S. was vulnerable.
The emerging missile defense system is making that less true, and a North Korean test launch is an ideal time to demonstrate that we are willing and able to defend ourselves.
UPDATE: Another proponent of activating missile defense.