There are NUMEROUS cases of folks shot with bird-shot who continued attacking. Bird shot only works on "non-committed, non-determined" attackers. A suicidal attacker that plans to die (domestic abusive husband or terrorist) or a person high on drugs (Miami face-eating zombie) can still kill you after several shots of bird shot. The miami zombie grunted after being shot twice by a police officer, and then went back to his meal, which consisted of the face/eyeballs of a homeless guy. You want to defend your home from that guy with bird shot?
You can use a way less effective ammo if you want, but I have PERSONAL experience dealing with suicidal subjects, and PCP/bath salt subjects. They are out there, in rather large numbers.
8 to 30 buckshot pellets blasting a hole through somebody tends to cause massive exsanguination (blood loss) which takes the fight out people with a quickness. Even those folks that are high as a kite, or suicidal. No REAL expert recommends the use of bird shot, for good reason.
People that say that a person would not continue their attack after getting shot, would be right most of the time. Most times, a person after being shot would quit, and exit the premises for a better ambiance. But, they do not take into account suicidal or drugged subjects. Ask any street cop in an average size city, and they will tell you that they deal with volatile suicidal subjects, or volatile drugged subjects on a weekly basis. This inst a very rare event. It happens daily, all across America, all the time.