It wouldn't have. In case of an all out war each side would send dozens if not hundreds of rockets at each target. US in particular had the problem where each arm of the military(airforce, army, navy and missile command) had their own and completely independent launch plans. So each one would launch their own compliment of nukes at Russian cities. Destruction of your own missiles by the already exploding weapons was actually seen as a significant issue that would lead to loss of many if not majority of weapons. Eric Schlosser's Command and Control is, as always, a great read about this.