Trident vote - what for ?

So Blair has won his Trident vote. I can hear newsnight in the background with Kirsty (I'm a real leftie) Wark leading up.

Some of the Labour MP's wanted to delay a decision - they have local parties to appease.

I'd assumed that Blair wanted the vote out of the way before Brown got in. However one of the few clear commitment Brown has given is to Trident.

So why have a vote now ? Given that in the future there will be less Labour MPs - its hardly going to get more difficult is it ?

Well perhaps you know something is coming that would make this vote very had to win afterwards ....