It's hard to miss someone coming through a narrow door a couple feet away.
If an FFDO did miss, I'd rather put a couple of missed shots in the forward lav or galley than let the plane hit a building and kill untold numbers (including me!).
FFDO's are trained to do the job. I was cop before and I've never fired my weapon on duty. FFDO's are just as ready to be in a "lethal force" situation as any LEO is. It's really kind of simple compared to street situations. Bad guy is trying to get in the cockpit and take the plane down; I'm going to stop him. I wish it was that easy as a LEO. There are hundreds of different situations on the ground and some of them are lose-lose situations.
I do agree there are some people/pilots who should never have a gun. I hope the FFDO screening takes care of that. I know there were lots of cops who shouldn't have them. I worked with them!