[vacri]

It's hard to say how many terrorist events security actually stops. Raising the bar reduces attempts, and who knows how many there otherwise would have been? It's similar to when police departments get more bodies pounding the pavements, as the mere presence of authority helps keep things in check.

Certainly there's a sweet spot and the TSA is on the far side of it, but it's not as simple as looking at a number.

[tikhonj]

I think that would be more of an issue if terrorism was a spur-of-the-moment crime. Violence on the streets can be completely random (violent people getting angry for no appreciable reason) or without much premeditation (a mugger mugging whomever he sees). In these cases, a police presence would deter a lot of the crime.

A terrorist attack would have to be planned. Anything at all meaningful is going to require explosives: you won't be able to hijack or even severely damage a plane with a knife or even a gun these days. Anybody who goes out of their way, gets explosives and probably has special training isn't going to be stopped by a bunch of ineffectual bureaucrats in silly uniforms. Anybody else isn't going to be much more dangerous on a plane than on the ground, so added security would make little sense.