Quote Originally Posted by Captain Fishpants
The logic of a shieldbearer is that the general is harder to kill, but his troops regard him (quietly, among themselves, when talking about the boss around the ancient equivalent of a water cooler) as hiding behind others rather than taking his chances like a real man. Hence the small morale impact. The +2 hitpoints for a general is very useful, though.

Quite straightforward really.
I think that logic is faulty. The army doesn't want to loose it's leader, so protecting him should improve morale. Alexander the Great had a shield bearer, and I've never see the idea expressed that this lowered his army's morale.