Its a sad thing to say, but i don't doubt that nuclear deterrence has prevented another major war in the cold war phase. I once read a text passage:
"Man is a hunter, a killer. We build great cities, and yet we live just like the wolf. The strongest of us dominate the weakest. We might call our leaders kings or generals, but the effect is the same. We create the wolf pack, and the very nature of the pack is to hunt and to kill. War, therefore, becomes inevitable."
While i don't see our civilization that negative, history has proven time after time, that peacefull societies sooner or later get attacked and destroyed by more aggressive ones. This happened regularly up to a man called Adolf Hitler, who started the last great war. Since ancient times the only defence against outside aggression has been a large military power of your own. There is an old roman saying: "Igitur qui desiderat pacem, praeparet bellum." Vegetius, which means, that those who desire peace, have to prepare for war. So the best defense is to build a strong offense. On the other hand, once you have a strong offense, it is tempting to use it, and eventually it will be.
The nuclear thread stopped this cycle at least in the western world. There where still wars, Vietnam, Korea, etc., but the major powers always tried to avoid direct confrontations. Of course this is a false sense of security, as MAD relies on responsible people in positions of power. It might have saved millions of lives, but if anything goes wrong, billions will die. And there where quite a couple of times when it was almost over, the Cuban missile crisis being the most famous example. I don't know what it says about our civilisation that only the thread of total annihilation can keep us from each others throats, probably nothing good.
There was a time i thought that at least the so called developed, civilized countries had outgrown this stage, the need of nuclear deterrence to keep peace, but then the Iraq war happened. A war based on lies, of which i am sure that the people responsible knew from the start. Personally i think the main reason was to distract from the fact, that the government completely and utterly failed to prevent the destruction of the world trade center. And it even worked. Bush was reelected after all.
If you would ask me if we need nukes right now, i would say no. As long as our bellies are full and fuel is still affordable everyone is happy enough. But once, in another 20,30,40,50 years, when the ressources start seriously to run out, we will be right back at each others throats, if history has teached us anything. At that time the task of keeping peace might very well be again laid upon a large nuclear arsenal. Even if most large powers wanted peace and would throw all their military over board, it just takes one strong nation to take advantage of the others weakness. Its mind boggling what amount of ressources we waste day after day to keep our armies, espionage and surveillance running, but thats seemingly the price we have to pay for peace.
My hope is, that one day we will develop to a technological stage, where power and other ressources become cheap and plentiful, as i doubt that we will develop socially to a point where we can let our differences and military rest and all pull on one string for the good of mankind, any time soon.