It seems to me that if he wanted to cause harm, he wouldn’t do it with something he brought onto the airplane; he would do it with the airplane. Or are they looking for something else?
The most compelling reason I’ve heard is so that they can catch terrorists who are dressed up as pilots or perhaps to catch pilots who have been coerced into taking something through security to give to someone else in the terminal. But yes, if a real pilot wanted to cause harm they have a crash axe, and in some cases a gun, in the cockpit to subdue the other pilot and they’re in control of the aeroplane itself of course.
Well, if pilots aren’t screened, then they theoretically could pass a weapon to a terrorist on the other side of security, which the terrorist would then use on a *different *flight. In this scenario, it could be quite a while before anyone figured out how the weapons were getting through security. That’s why if anyone is to be scanned, *everyone *must be scanned. Otherwise, you might as well not bother.