Islamic extremists are not the only security threat to the USA. We continue to exist in a relativel tense peace with China, Russia, and other nation-states.

The ability to drop a bunch of nukes on someone on short notice is not to deter terrorists, it's to deter nation states - anywhere between North Korea through Pakistan.

The existence of nukes and the submarines that carry them is almost entirely orthogonal to the (relatively new) thread of terrorism.

No, that's what the NSA and CIA is for. :P

I realize it's a circular argument, but MAD is for other nuclear nations (or future nuclear nations).