I have seen a Taser demonstrated. Its use requires considerable skill, certainly much more than using a handgun. It is also a one shot device, in that if you don't hit your target first time, he gets a chance to hit you. I'm with Aviator. If my grandmother had been on one of those flights on Sept 11th, with a handgun she would have saved the aeroplane. Will it take another incident before the politicians stop posturing and recognise that, as a last line of defence, the handgun is effective and simple.
As for security being our last line of defence, don't make me laugh. That is not to denigrate the security personnel, who do their job with diligence. But if I wanted to get a sharp object on to the aircraft, I would slip it down the spine of my briefcase. Short of tearing everyones' bag apart, it would not be found. However, getting a gun on board, now that would not be so easy. So if the flightcrew had handguns, chances are they would be the only people on the 'plane so armed. That is how I would like it.