[quote=Cyclone]
I'm not sure I entirely agree with you there Gecko. It doesn't necessarily limit Keeper types; a necromancer could keep human slaves around because they're bound to be smarter and in some ways more useful than mindless undead automatons. Even Sauron had human slaves. Sure, a fake benevolent Keeper would have the advantage of happy slaves, though this comes at the disadvantage of having to maintain the facade of being good. That means no terrifying monsters running around, no alliances with the other Keepers that are a bit more open about their intention
[/quote]

You could also have a Spirits Keeper, who's military is based around people possessed by spirits. Which adds another layer to the need for humans.