[Soyweiser]

Also, priests are more defensive, not really useful as an offensive weapon.

It also totally depends on the type of undead, some can spam reanimate in combat. Others (ma ermor, la ermor) summon large hordes of undead outside of combat, and throw hordes and hordes of these into combat. (Usually your small groups of priest are easily overwhelmed, flanked, crushed, evocated into oblivion).