I think you are too pessimistic.
Yes the system would fail miserably if hit with too many bogey's at once. But that is true for ANY defensive system.
Keep in mind that every one missile stopped might save 500,000-1,000,000 lives.
Hopefully the system will never be used (and history can then call it a waste of money), but we do need to explore the possibilities here because it will help us develop better defensive systems accross the board (anti-aircraft, and theater based anti-missile defensive systems are an example).
The better our defenses, the less likely we are to be attacked.
Scott Evil