Costs of an attack
It's also worth noting that a nuclear warhead flattening a city creates other costs besides simple loss of life.
As I posted elsewhere, it is estimated that the cost to New York City alone of the WTC attack is running close to $100 billion.
Imagine an attack that takes out hundreds of buildings, and makes an entire city center useless for years. How much is that worth?
Missile defense on several technological fronts is worth a robust research program at the very least. It is what the military is intended to accomplish, and a purely defensive weapons system is a refreshing change of pace.