Problem is that if you are fighting 3 of them you can't very well put your weapon away if the first one yields. That first one will crawl away, regen health and then rejoin the fight. Might work well enough on the last one. I have never seen an NPC completely break and run like they did in Oblivion. That would be nice.
All above posters who are confused about NPC's running away obviously don't pay attention or don't fight in a variety of ways or something, because I have people yield to me and run away all the time....
I certainly do have the ones who get injured and then return to fight mode if I don't Mortal Kombat them in time, but there's also plenty who freak out and run away and hide. However, I never put my weapons/spells/bow away until nothing is left living within my HUD range, so I don't let anyone live to tell the tale.