The healing spells give energy to the body in order to repair damaged cells. Excess energy infustion provides our character's bodies enough energy to continue to block, dodge, evade, and if we're lucky the Cleric's deity grants us a little divine protection/luck to boot! If you are fighting at 1hp you are in desperation with all or nothing. That commoner with 1hp is a very weak and frail man even when healed fully. He doesn't have a lot of endurance obviously. He'd die quickly in a fight for his life, and excess energy wasted because he wasn't skilled enough to use it. You don't take the mortal wound until you reach 0hp and should be described as screaming for mama, your guts hanging out, and you body flopping like a fish.