Iran is using the nukes as leverage; much as the Soviet Union built nukes but had no plans to use them unless war broke out. They're smart enough to know that they'll be destroyed if they actually use a nuke on the US or Israel.
MAD only works as a deterrence if both sides agree that destruction is a bad idea. A secular viewpoint would of course generally lead to agreeing to that. But Iran is not a secular country and its leadership in particular rejects secularism. While I'm not one of those alarmists who thinks Iran intends to use the bomb as soon as it gets it, neither am I polyyanna who thinks they'll only use it as a bluff.
The most likely situation in which an Iranian nuke gets used is if Iran ends up in a situation similar to Syria today. I can easily see Khamenei or his successor authorizing the use of Iranian nukes in such a situation so as to find out if nuking Teł Aviv would be considered equivalent to the sun rising in the West that is one of the signs of the end times in Islamic eschatology.
The only way to prevent that from happening is to keep Iran from having nukes.