MAD
I actually believe nuclear weapons of mass destruction and MAD have saved more lives than we could imagine, and by this I do not mean their role in bringing on the cessation of hostilities between Japan and the US after Hiroshima and Nagasaki but rather their effect on the world from 1949-1990.
The Cold War was indeed a dreadful trial for all parties involved and doubtlessly it came close to going very wrong, several times, most notably during the Cuba Missile Crisis in October, '69. Truly, I don't think anyone involved or reading the history books years from now would ever consider it a pleasant time.
But let's for a moment consider the repercussions of a Cold War sans nuclear weapons. Most likely, it'd never have time to even get cold. Korea, Viet-Nam, Afghanistan would all be full-scale battlegrounds where the superpowers waged their wars. Not limited as it would be for the fear of nuclear reprisals, but rather it would be total war, and destruction would be just as assured as if a nuclear weapon was tossed either way, and it would be, if not as showy on the news at nine o'clock, it would most certainly be more messy than the cauterization of a nuclear blast (of course, down the line, nuclear fallout would claim its toll, but recent studies from both Chernobyl and Japan show that these dangers have most likely been somewhat exaggerated, if not greatly). And I very much doubt the wars ensuing would stay contained as they were in the Cold War as we know it, rather they would flare up entire regions, and god knows we'd have this third world war that was so feared throughout the latter part of the twentieth-century.
I'm not saying we should all start loving the bomb, but we should look at how the scenarios we faced back then when the superpowers waged their little skirmishes would have played out without the threat of certain destruction if one party overstepped the silently agreed on boundries of aggression.
Truly, the possibility of nuclear war is a ghastly thing to even conceptualise, but the threat of the nuke is the greatest weapon we've ever invented thus far. And by far the least lethal.
However, MAD is a moot point in a day where nuclear capability is no longer out of the grasp of not only rogue states, but organizations with little less in mind than the complete destruction of their enemies with little danger for personal repercussion and reprisal. But as long as we stick to our great big wars where billions face billions, we're safe from the mushroom cloud as long as we choose not to use it. In the case of one versus billions, we're totally fucked.