A defense implies one is expecting an attack, and also implies (to some - the most dangerous ones, probably) that they are now open to a pre-emptive attack themselves. If the US has a good missile defence shield in, say, 2015, Jong Il Hussein, the pres of NK, thinks to himself "Yikes, all those missiles and warheads we built and now the US has a shield! They can blow us off the face of the earth with impunity! But wait - we're about wasted already, what do we have to lose, for the glorious revolution let us launch NOW! If even one or two of our warheads get through the shield, it will be worth our martrydom! And besides, the shield was built with the best of American technology, just like their Dodge Dakotas.

Impossible, you say?

Yeah, if NK is such a problem, just invade them and take out their nuclear program.

IMO an imperfect shield is worse than none at all.