I'm sure I'm hardly the first person to point this out, but ...

Your argument presumes that an air crew member would be trying to destroy their own aircraft. What if an air crew member was trying to destroy some other aircraft? All they have to do is walk through the hypothetically-non-existent screening with a weapon or weapons. Once inside the checkpoint, they hand off the weapon(s) to a co-conspirator, who boards a completely different flight with those weapons and subsequently causes an incident.

And, of course, this argument works even better if the air crew member is being blackmailed or otherwise coerced into complying with the terrorist in question. ("Help me destroy my plane or I'll destroy yours instead.")

Supposedly, the point of current TSA screening procedures is to keep WEIs from getting aboard aircraft. TSA attempts to achieve this goal by trying for a much more difficult goal: keeping WEIs from getting onto concourses (the so-called "sterile area"). As long as there are people who can enter the sterile area without being screened for WEIs, there are obvious ways to get WEIs into the sterile area, and thereby onto aircraft.