1. The concern is not primarily for conventional missile delivery of nuclear warheads (Israel aside) but rather for small "suitcase" style nuclear weapons in the hands of trans-national terrorists, supplied by Iran. MAD doesn't apply in this scenario.
2. It is possible that Iran may have a state suicide mentality and MAD would not be a deterrent to it. Secular states (like the U.S. and the old Soviet) were much more likely to respond to the MAD principle.
