I think that if we're going to have a nuclear deterrant, we should have one that can't be taken away from us on the whim of a foriegn government. Regardless of wehter the UK has operational control of Trident (and that's debatable in the absence of any UK TACAMO equivalent), we're wholly dependant on the US for missile and warhead design, missile development, testing, maintenance & support, warhead testing, submarine degaussing, etc. If the US decided, for whatever reason, that it no longer wanted the UK to have nukes, it could make it very, very difficult and expensive, if not impossible, for us to continue.
Also there's the issue of basing - can't see an independent Scotland being too keen on keeping them.