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, and no alliances with the other Keepers that are a bit more open about their intentions. You would have to be very careful so as to not do anything even remotely evil or suspicious, so as to maintain your guise.