Another thing to look into with Iranian Nukes is they unlike the US and Russians will never have enough weapons for MAD. So yeah, I think they are going for a trump card.
A couple nukes isnt good enough to "Hold the world Hostage" but it works really well to keep people to mind thier own buisness. Its sorta like holding a gun on a crowd of people trying to jump you, yeah theirs a lot more of them then you have bullets, but some of them know they are gonna pay in blood to make you comply. So it becomes a game of is this really worth it.
Also I dont believe the Iranians would be crazy enough to actually nuke the Israeli's or us for that matter. Yes the country and the people hate them and us with a passion, but at the level of the political powers that control those weapons they are smart enough to see that terrorism with B/C/N weapons would be instant death to any regime that sponsored it. They only have to look next door to see what happens when a Super Power believes that you are sponsoring weapons to be used against it, much less if a bomb actually went off. Also the Israeli's arent really known for taking things up politically when they feel threatened, just look at Osirack.