There are about 2,000 armed air marshals who fly on domestic and international US flights as passengers.
Always thought that armed sky marshalls should be trained pilot's, lets say the hijackers did make it to the cockpit and eliminated the capt and f/o, the sky marshal then eliminates the hijackers, what do we do now? If he was a pilot it would give the chance of getting the bird down.
I suppose the sky marshall is there to prevent the HJ's getting near the cock-pit.
Still it could be a way for rookies to get at least a type rating out of it and subsequent currency training.