The ones who really need an anti-ballistic missile system are the Israelis, and they already have their Arrow system deployed. For the rest of us, a nuclear attack would be more easily delivered (by a "rogue state") via a cargo container carried on a ship. This vulnerability has been recognized, as proposals have been made to scan all seaborne cargo containers entering the US for nuclear material. The proposal has been stalled because of the sheer size of the endeavor, and the cost. There are also technical questions as to whether it could actually be done.