Yeah, shields wouldn't really have that great of a damage capacity (outside of spikes or other implements stuck on them). Unlike another blunt weapon like a mace, which sends the entire force of impact into a small area, shields would spread the force of impact through their entire frame, so even larger shields wouldn't do near as much damage (possibly even less than a smaller shield, although they'd have more weight behind them). They'd be better for knocking the opponent off their feet and maybe breaking some ribs in the process if you can put enough force into it. Dual-wielding shields would give you a lot of defense but not much offense.
Of course in a numbers game like WoW that doesn't really have anything to do with how much damage they'd deal (Shield Slam does way more damage than Prot warriors' weapon attacks).
Even then, realistically, swinging your arm in such a way with that type of surface weapon takes a lot more strength and would wear down your energy much more than simply swinging a weapon where it relies on it's weight and the difference in the way you swing your arm compared to swinging with a shield.