Not only that, but she ignores the hard-to-quantify deterrence effect -- thugs who want to commit crimes against persons, but don't because they fear the person might shoot back. The gun is not "used" at all -- it's not even drawn. The mere possibility that it might be there stops the crime from happening.
Muggers, carjackers, armed robbers -- they commit a LOT of crimes per year if that's how they support themselves. If you commit two hundred violent crimes per year, and just 1% of your victims are armed, your luck will run out before end of year.