As lots of posts imlply, it really seem to be a cultural thing.
Lots of Europeans (myself included) believe its a bad idea and contributes more to endangering aviation than making it safer.
Lots of US-Americans believe its a good idea and feel comfortable with it.
I personally find the idea absurd, but I have to accept that people in the US grow up in another mindset towards guns in general.
To share a flightdeck with an armed pilot would make me feel uncomfortable.
So what? The chances that I will ever fly on a N-registered airliner arenīt very high, so I donīt really worry about it.
I still donīt get the concept. A group of terrorists try to pull off the 911-thing again. They are well trained, prepared to die and have the moment of surprise on their side. They could use one of the cabin crew members as a human shield upon entering the FD. If you kill the first to enter, there may be five of them left.
I believe that the downsides of bringing a hazardous equipment onto the FD outweigh by far the small advantage you might or might not have in another terrorist attack.