They are more malleable, easier to command, and capable of following simple orders. Plus, if you use minor Daedric spirits to animate them, corporeal undead have no sense of self or memory of past lives. Corporeal dead provide more muscle, which comes in handy when you need manual labor or an instant fighting force. Vengeful ghosts, of course, and other types of incorporeal spirits abound, but they have little substance and can often be difficult to control.
Many options present themselves for the industrious necromancer who wishes to raise undead minions to serve or protect.
