Oh, really Senator Hollings? So what happens when a hijacker holds a box cutter to the throat of one of the passengers or flight attendants and demands the door be opened? Wouldn't it be nice if the door opened and the hijacker was greeted with guns a-blazin'?
No. A pilot's job is to keep control of his airplane. For the pilot to intervene in the cabin would be to risk losing control of the aircraft.
As long as the pilot remains in the cockpit, the only possible direction of attack is through the door, and identification of friend/foe is easy. Essentially, the pilot just has to make a center-of-mass shot at any unauthorized person forcing their way into the cockpit. Not exactly a task requiring a high degree of marksmanship or expert judgement. Nearly any situation involving intervention in the cabin will require far greater skills in marksmanship and friend/foe identification. Given the possibility of a hijacker posing as a "friend" who assists in subduing other (decoy) hijackers, pilot intervention in cabin disturbances is a recipe for disaster.