It seems reasonable to me that the number of weapons taken through security should be monitored. People are licensed to take a gun through, and a check can be made that said gun is still in their possession when they leave the secure area.
If someone comes along with an extra weapon and are let through, what procedure monitors that they still had it when they left the secure area?
Allowing someone through security with a specified weapon is quite different from allowing them through with as many weapons as they like.
If Al Qaeda are the masterful covert operatives we are told they are when the state needs to justify removing our civil liberties, we should certainly fear that they can bribe a policeman or armed pilot to carry a weapon through security for them.
If they are the bumbling oafs who get occasionally get lucky when we take our eye off the ball, this isn't the most intrusive unnecessary measure that is currently deployed.