Style and realism! In real life even a light injury is debilitating for an average person, because of the psychological and physical shock.

I recently wrote rules to cover muscle-powered and early black-powder weapons.. I was interested in incorporating armor penetration into the equation more. Instead of injuries I have a continuum from hits: stun-minor-injury-major injury-death, modified by skill, strength, weapon type and range, armor, and defender's constitution. Con also gives you a chance to avoid shock from injuries.

Ever play Champions (Hero Games)? I liked their Stamina/Body v. killing/non-killing attack setup.