Capt Pit Bull,
The point that has been made previously is that deterrence only works if your adversary considers you will use your nukes.
So therefore, it is logical to analyse when such use would take place.
If you can state that the weapons would never be used then they fail to be a deterrence.
Your "analogy" about fire brigades is logically incomplete since it fails to take into consideration the times when the fire brigade are used successfully.
The fire brigade is designed to be used yet you are comparing them to a weapons system you yourself state is designed not to be used.
This is as flimsy an analogy as those who say Trident is like an insurance policy. You hope never to use it but you like the fact that it is there.
This is false too since an insurance policy is designed to return you to a similar state to the one you were in before whatever accident befell you.
It is not designed to burn down the rest of your street if you have had a fire!