Anti: Nuclear weapons are awful, we should make a positive gesture towards a more peaceful world and get rid of them.If we accept the premise that the current geopolitical situation doesn't warrant the UK having an active nuclear deterrent (which seems reasonable) but that this situation could be subject to change (which also seems reasonable), a warhead free submarines plan could potentially satisfy the needs of moderates on both sides of the debate.
Pro: That's naive, we all agree that nuclear weapons are awful but other nations have them and might use them, so we need them as a deterrent so people know that if they shoot at us, we can shoot back.
Anti: But the world has changed, in the cold war we always worried about the USSR, but its gone and Russia doesn't really look like it's going to try anything these days and other nuclear armed states have neither the capacity or the desire to attack the UK.
Pro: But the political situation could change, although the situation looks reasonably benign now, it could quite easily get worse with a few political changes.
If we were to keep the submarines it would mean the UK would retain the ability to re-arm if global geopolitics warranted such a change. An awful lot of this depends on specifics: Do we keep the warheads in storage? Do we dispose of the warheads but maintain the ability to produce them in future. What this debate amounts to is a debate about how long it should take to switch the UK's nuclear deterrent back on, if we throw the off switch.
Ultimately, what we have here is a way of providing a step in what I think can universally be agreed is the right direction without ruling out a return should the situation warrant it. It seems like a reasonable argument to make and it's a depressing indictment of British politics that what appears to be both a reasonable and pragmatic suggestion is dismissed so readily and vehemently.