My companions keep finishing those who attempt to yield and I read that it only affects certain enemies like bandits anyway, while any other types just get up and attack you again [the liars!].

If it hasn't been fixed I hope Bethesda get around to it [in 1.9 patch or whatever, though its not game breaking by any means].

They're not exactly yielding, those are just the phrases they utter when their HP is low. NPCs are incapable of yielding. Either they run away immediately or they fight till the last drop of blood.

Yeah I wish when a bandit yielded he would actually ya know... yield. Perhaps some could become generic followers, would make the game a whole lot more dynamic.

NPC's don't yield, only you can to them. Those are simply 'panic' lines given when the NPC's bravery is outweighed by the situation his is in, leading to him running away in fear and shouting those things.