I voted "No" and find the fact that so many people voted "Yes" in some way or other mindboggling. I am not convinced that any GM needs a rule about morale. At all. If a GM wants opponents to give up or run away, they can do that without any rule. Just decide that it happens... and it does! It really is that easy. Especially since this is completely an issue of plausibility that needs to be regulated, if at all, at the individual table in the concrete situation and story. Any morale value, however delicately designed, is unable to handle that in any meaningful way without weighing down the entire decision process.
I mean, if it suits your game, fine. Please create a houserule and argue over this in your gaming group. But I think this is a waste of trees. How much reality does one want to be prescribed by designers in one's own game?
To be honest, I cannot find a rule less german than a morale rule in a RPG.
True - the DM is the ultimate arbiter. But, having a simple rule system allows one to easily maintain consistency. ("Hey, those goblins ran away in one round two combats ago... why are they so stalwart now?") While I'd love to be a perfectly consistent DM, I can't remember every little detail of each encounter (gee, did those goblins run away after they lost 6 of their number, or was it 8?) Having an easy morale rule would make my life easier.