It is important to discriminate between pain and injury. From the standpoint of self defense this is pretty simple: injury reduces the physical ability to fight or pursue. Inflicting injury can cause pain as well (though not always immediately), but pain by itself does not take away an assailants actual ability to fight.

A perfect example of this is professional soccer. Players who have been fouled roll around on the ground writhing in pain until the referee assigns the penalty and then they jump back to their feet and continue to play the game as if nothing happened. I am sure that the foul was “painful” but if they were “injured” they would be getting carried off the field. Part of being a professional athlete is being goal-oriented and shrugging off pain to get things accomplished.

Martial Arts and self defense systems that rely on using pain to influence behavior with pressure points, arrest-control holds, and the like are using pain to get voluntary compliance. These methods can fail against people who have either a higher tolerance for pain or are impaired by drugs or alcohol. The pain only lasts as long as the hold is being applied, so compliance after being released from the hold is voluntary behavior.

Injuring the assailant makes him unable to continue the attack. Knocking them unconscious, breaking bones, and causing significant amounts of blood loss all degrade his physical ability to fight regardless of his mental state or willpower. “Superficial” injuries might have some deterrence value, but assailant gets to choose if he is going to be deterred or not. If you can treat it with a band-aid it probably isn’t an “injury.”