For minion creatures, or for large numbers of creatures, the averaging of damage rolls make much more sense. Against 5 goblins making 2 attacks per round their damage over 5 rounds, for example, will in principal average itself anyway. Standardising this process seems like very sensible behaviour.
For single powerful creatures, however, the variation between a string of 1’s and a critical threat is what makes the encounter super-edgey. In addition, powerful foes rarely last for 10’s or rounds anyway so that averaging the damage is completely artificial – they are unlikely reach average scores in their own short time so forcing them is misrepresentative.