We've been through this.
Most people who say it "can't work" have no idea on Earth what they're talking about. This board is a very rare exception.
My take on it:
-- If you don't work on building it, it won't ever get built.
--A system that works half the time is infinitely preferable to no system at all, which is the only alternative we are given by opponents.
--One single incoming warhead shot down would pay for the entire program at almost any price.
And I ask the usual question: Why do some people want so badly for it not to work?