The original IE games, such as Baldur's Gate and Icewind Dale, implemented a strictly health=live, no health=death mechanic. In these rpgs you are fighting monsters who crush and shred your flesh with tooth and claw, spellcasting wizards who with arcane energy can immolate or rip the life right out of you, duelling brigands and orcs with maces and iron swords. You suffer injuries of the flesh, represented by health. Perhaps the most exhilarating and horrifying aspect of the original games, was the character portrait that would fill up with crimson, from their neck up, as they their bodies were being torn apart by demons and werewolves, and then.... death. A black and white portrait, all colour drained from their normally lively faces.
Why then, would we have Endurance? You are fighting to the death with monsters and other humans. You do not simply fall unconscious when crushed by the fist of an Iron Golem, you are not simply knocked out after a Beholder casts Finger of Death. You die.
I believe that Pillars of Eternity would benefit from this mechanic, not just for realism, but for the feeling of despair and revenge after each of your party's portraits fill up (not down) with red, accompanied by their agonized sounds as they are dealt damage, ending with a black and white portrait.
not to sound too morbid