Missile Defense’s Role in 21st Century Deterrence

James Jay Carafano from the Heritage Foundation pubilshed an oped today discussing the role missile defense can play in the altered deterrence environment of the 21st Century.  Citing the lack of deterrence to prevent likely pressing nuclear dangers (terrorism and rogue states), he argues that missile defense can operate as another layer of insurance to discourage potential adversaries from planning a complicated attack against the United States.   A large part of Carafano's endorsement for missile defense relies on his threat assessment that the typical scenario of a dirty bomb type attack is not as likely as assumes whereas a terrorist attack using missile may actually be more feasible.  He explains, 

In short, it’s unlikely there are many terrorists out there with the smarts to put a bomb in suitcase -- and it’s unlikely there are any to buy, either . . . A truck-borne small “real” nuclear weapon detonated in downtown New York might kill 40,000. The same weapon detonated as the warhead of a missile in a low-altitude airburst might cause half-a-million causalities. If you wanted to send a message to America, which attack mode would you chose?  And missile threats are not out of reach for terrorists with even modest means. Short-range ballistic missiles can be bought on the open market. They can be launched at ships from sea that never see an American shore or come near a Coast Guard cutter. 

Is Carafano's assessment of missile attacks being more likely and container attacks less likely correct?  Can missile defense play a role in deterring terrorists from planning a nuclear attack?