Their crappy missile would most likely fall apart during flight or completely miss whatever they're aiming for. But given the possibility that they actually did connect what they were aiming at, suppose they hit a large metropolitan area? In addition to the the immediate damage and casualties, what would that do to government services, banking institutions, and communications infrastructure throughout the rest of the country? What about the breakdown of order something like that might cause? Looting, rioting, burning, etc.
Hope everyone has stocked up on ammo, water, potassium iodide tablets, and Hostess Twinkies.
'throughout the rest of the country'
A single nuke would be devestating for the place it hit, but unlikely to cause any real problems in the rest of the country- except for the fallout.
Government services would continue- even if DC were hit. Banking institutions would continue even if New York were hit.
Wouldn't be pretty- but the United States is bigger than a single nuke. And of course it would immediately put America on a total war footing- with massive government spending and massive government employment.