A nuclear deterrent gives us the ability to defend ourselves and (as it says on the tin) deter those states who have/are developing weapons, from using them.
So we might as well let Iran have their nukes then. After all, they've threatened nuclear powers (Israel & USA) so, assuming that MAD is so secure, Iran won't be using any nukes they develop. If they can't use them, why not let them have them and place a huge financial burden on their country to no benefit.
Of course, MAD may not be such an assured path to peace, which would explain why the world is so worried about Iran...
