Easy.
WE have proven that we won't use them even when under considerable stress from others (Cuban missile crises, anybody?) Our having them has prevented other major nations from having them.
Yes, we used two of them when they were first developed and we didn't have a real idea of what those things would do, but even then the use saved a bunch more lives than they took. Japan WOULD NOT have surrendered with any lesser action. Hundreds of thousands of OUR soldiers would have died to prove that, and Japan would have been utterly wiped off the map. Japan...had, and still has, a warrior culture that 'to the last man, woman and toddler" was an absolute. It's one of the most admirable things about them, when they aren't targeting US, that is.
Iran...and other nations which support terrorism, would. Any group which supports actions like 9/11 and the other terrorist acts absolutely would use them. That's pretty obvious. It's also VERY obvious who would get targeted by anybody like Iran..(and other nations which support terrorism). Self defense is a strong motive.
But finally, we can say that because we got them first, we have the power, and we don't want little dictatorships who hate us to have the ability to attack us with them. Because they would.
My opinion, but I've been around awhile, and I don't have a good opinion of human nature.