On that subject, turn up your speakers and...
click here. Now tell me, if you were a passenger with bad intent on that plane, would you really try to start something?
On a more serious side, the whole issue is to provide the cockpit crew with a final line of defense. They aren't going to leave the cockpit for anything anymore but, should someone succeed in breaking into the cockpit, it'd be nice if the crew had more than the crash ax to keep someone from gaining control of the plane.