The danger is not NK shooting a nuclear missile at us, but rather, providing nuclear weapons to terrorists. The NKs are using nuclear weapons to blackmail the West and Japan to get material assistance. It has worked in the past and will seemingly work again. This pissant country has not even tested a nuclear weapon.
NK is not suicidal. Launching a nuclear weapon against the US would be suicidal, and Kim Jong-il is a survivor who worries about his personal safety every day of the week.
I agree with your assessment that NK giving nukes to someone else (or creating "someone else" out of the whole cloth to hide behind) is more likely than a direct attack on an American city, but what about a "test" detonation above the atmosphere? They are not a signatory to the treaties banning the testing of nukes nor specifically their detonation in outer space. Since we and the Soviets did such tests, how can we complain if they do the same?
The fact that doing such a thing now would result in an EMP that would wipe out the electronics and most of the electrical grids within direct line of sight isn't their fault, is it? Could they weasle out of that one? More importantly, could they THINK that they could weasle out of that one?
If we didn't respond immediately to such an event can you imagine the United Nations debate? What about the damage to our society (or Japan or any other technologically advanced nation)?