To take the other side (not sure I agree):
Iran is unique in that it is run by a government that claims to be Islamic fundamentalists. This could make mutually assured destruction less effective if the ones launching the nukes truly believe that if they die in the retaliation they will go to heaven with their 72 virgins.
I'm not convinced that the Ayatollah is ok dying in the name of killing infidels, but I do see the merits of this argument.