The scientists have been consistently open on this issue, ever since its first incarnation under Reagan. They're the one's who'd know. Who am I to say otherwise. Here's a recent quote.
"'This is perfectly reasonable, since a non-working missile defense system should be sufficient to deter a non-existent threat from Iran or North Korea.' Bob Park, editor of the American Physical Society's "What's New" newsletter, on the US Senate Armed Services Committee's decision to deploy the missile system without independent tests to prove it works (18 June)"
Quoted in New scientist, Vol. 182, no. 2453, 26 June 2004, p. 12