[Poohbah]

Terrorists brandish weapons, citizens respond, gunfight ensues.

Terrorists know who bad guys are; citizenry doesn't, causing multiple blue-on-blue engagements.

[Shryke]

That's pretty much the basis of my argument. Allowing everyone to carry a weapon would NOT reduce terrotists threat to aircraft. It would increase the threat. It's a question of initiative (terrorist is going to open fire first) and liabilities (terrorist WANTS everyone to die).

[El Gato]

Terrorists know who bad guys are; citizenry doesn't, causing multiple blue-on-blue engagements.

Could happen, although not so terribly likely. It could happen that an air Marshall would shoot the wrong person as well. AFAIK, that hasn't happened yet either. Is that worse than letting the terrorists crash the aircraft into a building and killing thousands?

Still, small steps would be OK as well. Implement the armed pilots law in a manner to encourage pilots and other aircrew to become armed, rather than to discourage them.