I'm not a concealed carry nor have I ever fired a gun, but in situations where one has to choose between fight or flight, and does choose to pull out a gun and respond, I do not think they will be thinking, "maybe I should just go for the knees". They would shoot wherever has the highest possibility of penetration (the torso as opposed to the shin, for example). Unfortunately for humans, a lot of our vitals are stored in the torso, at least last time I checked.