The purpose missile defense, should it ever gets up and running, is not to defend against Russian missiles, or Chinese missiles. The idea that such a system would work against the Russians is absurd. The purpose is to prevent "rogue" nations, such as Iran with limited nuclear and missile technology from setting up a nuclear umbrella, which would deter the US from attacking.  In this sense the purpose of missile defense is not defensive per say, but offensive.  There is no nation on earth that would ever attack the US first.  The purpose of the system is to prevent some “rogue” nation from firing off a missile when it has nothing to loose or to remove the ability of such nations to threaten such actions. 

With regards to the war on terrorism, it would also do something else.  Minus a nuclear umbrella, such nations would be detered from allying with the likes of Al Qaeda.  Al Qaeda would not be able to set up a base of operations under someone’s nuclear umbrella let alone carry out an attack on the US and then be able to scurry back under such an umbrella.