Why?

Gates supports missile defense despite budget cuts

The article notes that, though Gates is seeking to trim more than $1Bn from the missile defense budget, he's still a strong supporter of the program.

Why?

Long, long ago, this was one of the very few issues about which I had strong opinions.
I think the technical obstacles are close to insurmountable - coordinating detection with interception.
BUT: even if the system works like a charm, so what?

Two words: Maginot Line.
The Maginot Line worked perfectly - the Germans did not penetrate it.
They went around it.

Similarly, a low-tech cruise missile - basically a WWII-style V-1 - launched from a tanker off either coast is all it takes to defeat an anti-ballistic-missile defense system, and deliver a nuclear warhead to LA, SF, NYC, or DC.

Then there's the damaging psychological effect: the French thought the Maginot Line protected them, so did not invest in other, more robust defensive measures (e.g., well-trained standing army).
Similarly, an anti-ballistic-missile shield may lull us into a false sense of security, making us blind to more obvious - and more dangerous - threats... like V-1 missiles parked just outside our territorial waters.

Me? I think the best deterrent against attack by ballistic missile is what in fact seemed to work during the Cold War: the very real threat that we'll do our damnest to wipe out any country that launches a ballistic missile against us. True, we sacrifice a city. BUT - the attacker sacrifices his country!

Note: I do NOT share President Obama's dream of a nuke-free world.