There was a successful case of a hijacking on a Boeing 747 being stopped due to an armed LEO onboard.
Wino you may be able to help me on this. I can’t remember all the details but the hijacking occurred in the 70s on a Pan Am 747 (I think). The aircraft was on a flight in Southeast Asia. Some guy armed with a weapon of some sort hijacked the aircraft and demanded that the captain land in Hanoi. There was an armed New York City police detective onboard, the captain got the detective’s gun and killed the hijacker.
The airplane landed in Saigon where the captain threw the body of the hijacker out of the aft door onto the ramp (a bit much if you ask me, but what the hell). I can still remember seeing the picture of the body lying under the tail of the airplane.
So having an armed LEO onboard stopped at least one hijacking. Why can’t it work again? You don’t need to have Air Marshals on every flight, just the threat of them being on any flight will help deter a hijacking.
As the people on the pro side of this argument have stated over and over again, it never hurts to have at least one more line of defense in your favor.