Well, supposing motivation to totally destroy a country, allowing both to take out each others military installations may result in less loss of life that one side being totally dominant. Obviously either situation is beyond terrible.
Nuclear weapons (and all other sufficiently powerful weapons) really do suck.
Since it would be very hard, though, for nations to keep enough around for mutually assured destruction without anyone noticing (or so I think) I would think that disarmament is not an entirely crazy idea. I would even assume that an agreement by the US, Russia and China would be sufficient. (I’m not sure to what extent Pakistan and India can threaten the rest of the World and I really don’t think Israel and North Korea matter. I’m also pretty sure that France and the UK would follow the US because they would look pretty stupid if they wouldn’t.)
