Okay, it helps to note that the U.S. general in question is the guy "who runs the Pentagon's missile defense projects".
Assuming - for the sake of argument - that the missile defense system works like a charm (note: this is a huge assumption. In the lastest "test" the Navy shot down only one of two missiles!)... So what?
The Maginot Line worked like a charm!
The Germans never broached it!
They went around!
How were we attacked on 9/11? Commercial airliners.
Would a 100% functioning missile defense system have protected us?
A primitive cruise missile (e.g., a WWII vintage V-1 missile powered by a ramjet engine) launched from a cargo ship off the coast of California or New Jersey could deliver a nuke more effectively than currently non-existent ballistic missiles from Iran or North Korea.
Assuming - again a HUGE assumption - the system worked perfectly, it provides close to 0 protection from attack, at huge cost!
The bad guys can just go around (see, e.g., German invasion of France in WWII).
Maginot Lines DON'T WORK!!!
To quote my favorite military philosopher (perhaps not too accurately - the quotation is lifted from the movie, Patton):
"Fixed fortifications are monuments to the stupidity of man. If anything made by God can be overcome, anything made by man can be overcome."W's vaunted missile-defense system is simply a high-tech "fixed fortification".
Again, it is helpful to note that the General urging Obama to keep the missile-defense system is the guy in charge of the program!
Stop the madness!!!