Tactically, the countries that have the capability to hit us (e.g., Russia, China, etc.) have enough missiles with enough multiple warheads to get past a defensive missile shield (remember, only a handful of missiles need to get through and these countries have several times more missles than we would have defensive missiles).
The "renegade" countries with nuke missile technology cannot deliver on ICBM's. So, they would have to deliver a nuke by low-tech (e.g., plane, ship, truck).
Thus, I cannot see the tactical advantage of a missile shield.
But, I think there are 2 main "unspoken" reasons why our gov wants 'em:
1 - Plain and simply, intimidation. We will have something they (THEM) do not, another ace-in-the-hole, "We've out-teched your sorry butts and there's nothing you can do about it - WE RULE! Fear us, you Commie bastards!"
2 - I think there's a more nefarious side to all this. I strongly suspect the so-called "Missile Shield" will also have other capabilities (including "first strike") our gov will not disclose. I think there's going to be a way to tie in the Missile Shield with a satellite, computer-controlled, targeting system that will have the ability to precisely deliver nuke and non-nuke missiles any where we choose. Buried among all the $$$ being spent on this project will be increased surveillance and targeting capabilities. This Missile Shield will be the ONE SYSTEM to finally pull it all together.
But, heck - what do I know?
trask