What would happen if a bunch of guys try to take over the cabin. Then when the captain comes out with guns drawn, they bum rush him.
The hijackers then have a gun.
It is no different than the logic behind why some prison guards have no gun either...
We trust these pilots with our lives already. Why not give him the tools he needs to do that job better? Maybe when 70,000 people are killed we will awaken to this simple fact. We must be able to protect ourselves. The Federal government has already proven that they can not protect us. We seem to be forgetting too quickly that 7,000 Americans were just massacred because the pilots could not defend the cockpit.
It might be a little more impressive if you could come up with a senario where the gun increased the chances of the plane being hijacked and more people getting killed than actually happened.