Try to imagine the effects a nuclear detonation or dirty bomb in Manhattan. I believe most people are undervaluing the probability and results.
That said, such attack would obviously be a horrible thing, and reasonable preventive efforts are justified. The key word being reasonable.
What train of thought brings you to considering the destruction of a major city by a nuclear bomb a 'minor' event?
In no way I want to imply that such an event would not be absolutely horrible and an act of evil. But considering the actual damage that nukes do is what finally cured me of being scared shitless of nuke-wielding terrorists. And yes, I used to be really scared of that.