Missile defence is a waste of resources and money and just another toy for the military to develop and spend money on. Plus it makes the administration look as if they are doing something about national defence.
The likelyhood of a bomb isn't going to come from a nation with ICBM's due to the reason it hasn't in the past 50 years even during the Cuban Missile Crisis. It will come by terrorists and they aren't going to bother with an ICBM attack when you can simply UPS the bomb to anywhere in the world and set it off by a cell phone.
Unfortunetly this basic technology of the terroist leapfrogs that of any Missile Defence technolgy costing billions.
There is more chance today of a bomb nuclear,radiation or otherwise going off somewhere in the world then there ever was during the cold war and they certinly wont broadcast the event by launching a missile.
...-Gixer