There is another plausible reason to have a weapon such as the MOP that has nothing to do with targeting Iran’s underground nuclear facilities. It might also be a way to conventionally neutralize Pakistan’s nuclear arsenal in a worst case scenario where the Zardari government falls and is taken over by radical Islamists who support al-Qaeda and would have no qualms about handing over nuclear weapons to terrorists. Please don’t confuse this with calling for military action against Pakistan. But if the above worst case scenario became a reality, we might have to seriously consider such action. And it would be imprudent not to. However – unlike the Bush administration did in Iraq – we should not plan as though the worst case scenario is also the most likely and then act prematurely to prevent it from happening.

http://original.antiwar.com/pena/2009/10/15/the-mother-of-all-bunker-busters/