Well, I would say that the lack of atacks is because the real terrorists get the message. Namely, that trying to hi-jack an airplane is not worth the effort.
Hey, wait a minute.......That would mean that the number of hi-jacks in Europe, where pilots are not armed, should be on the rise. But as this is not the case, I would conclude that arming pilots is unnecessary from a deterrence point of view. (or any other point of view for that matter)