Nuclear missiles under the sea - WHY?

I saw the news of the French and British nuclear (or "nuculer", as the man from the admiralty said) submarines colliding under the atlantic, and just thought: WHY?

http://www.abc.net.au/news/stories/2009/02/17/2493107.htm

Why is the "nuclear deterrent" still deployed, when they have noone to deter from launching a first strike. i.e. The most likely threat is from a rogue terrorist organisation (does that include the Iranian government) who has no concern for a retaliatory strike. Are "we" really going to nuke Moscow if some Khazak islamists manage to launch a nuclear weapon.

Obama is making the right noises about major disarmament, potentially keeping "only" 1000 warheads (capable, presumably of killing about a billion people if aimed at the right population centres). What I want is the US to unilaterally dismantle ALL of its nuclear weapons, given that I can't imagine what targets they could possibly use them on... The cold war logic of MAD is now redundant.