While I agree that getting beat up would realistically lower ones combat ability, I generally play for heroic fantasy, not realism. Having played in game systems that lessen abilities as you get hurt I find it to be less enjoyable to play. The reason is it virtually eliminates the possibility of the heroic comeback. That is a common trope in stories. Think of Rocky or Die Hard. If they have multiple penalties because they have taken some blows, it essentially guarantees the outcome. Early advantages in a fight become insurmountable and the dramatic tension goes away.