Having some enemies flee makes sense (especially when you get down to the last 1-2 mobs out of 10-12) but it would decrease the overall XP the party can acquire from a particular area, which would make it harder for the devs to balance combat encounters further along in acts 2 and 3 - unless fleeing enemies still gave XP (just no loot).
If fleeing enemies gave 0 XP, then something like the old BG random encounters could be added added (ambushes in goblin infested areas, wolves or bears when camping in the woods, bandits in the mountain pass, various beasties in the Underdark) to help mitigate the XP loss.
+1
