Paulo
I see what you are saying, and do agree that ID cards can be faked, just like any other 'official' document that's ever been known to man! But El D has made the point that the rest of the crew are unlikely to be taken in by an individual who they have never met and who behaves suspiciously.
My point still remains though; security check every ID of persons going airside. Why do they check that ID? To ascertain if the person carrying it has authority to be airside. If they cannot be confident that an individual is who they claim to be then that individual should be refused entry and the appropriate authorities requested to attend. If they are, then there is no need to search them (when it comes to aircrew) because they now have access to the aircraft by virtue of their job and the authority issued to them. If they are intent on causing damage by crashing the aeroplane, removing a nailfile at the security desk does nothing to prevent this. Searching them to see if they are carrying an explosive device would therefore be futile too as they would not need to risk being caught carrying one................
So I cannot see a valid reason for searching aircrew, unless you suspect that terrorists have infiltrated an airlines pilot workforce........and if that's the case the security has already been breached.
regards
PP