I see what you mean, but the problem with having a nuclear iran is that they will use it against Israel, which is America's ally. The Iranian president has made that clear on several occasions. Also, his certain brand of extremism dictates that a world consuming war will hurry up the return of the Shaman (pardon me if I misspelled that) and bring about a Utopia on earth. So that nut actually believes that if he starts a nuclear war with America, let alone a general Middle-Eastern war, he will hurry up the return of that Muslim sects 'savior'.
So basically it's a crappy situation no matter what. And whatever happens, the only nation that will benifit in the end is Russia. Which will make the current chill in relations between the East and West plunge into a second cold war.