Dick,
The guns are not for going into the cabin, its to keep people from the cabin out of the cockpit.
Guns in the cabin would be a bad idea, because those could be commondeered and fired into the cockpit. However, guns in the cockpit could be pulled out in response to an assault on the cockpit door and anyone that comes through the locked door could be reliably dispatched with minimal training.
As to security keeping knives off of airplanes, they can't keep knives out of prisons where the prisoners have no rights are searched (strip, cavity search etc) on a regular basis. If you can't keep knives out of prisons where you have total control over the inmates, you are kidding yourself if you think you are ever gonna be able to keep them off of aircraft. Furthermore, entire prisons have been commondeered by the prisoners (attica for example) which is much harder to do than commandeer a plane.
The LAST line of defense will always be the cockpit. Does anyone really think that things would have been worse on 9/11 had the pilots been armed?
Cheers
Wino