After reading the comment from Lexx in this thread,
http://fodev.net/forum/index.php?topic=16287.0I started thinking about slaves. They really shouldn't stay put and do work for you if there's a chance to escape. You also shouldn't be able to have tons of slaves with weapons with you being the only thing between them and freedom. So I propose a slavemaster merc that you can hire from Vortis or Metzger. In order to leave slaves anywhere, you'd need to leave this guy there too. As long as this merc is there and not dead, your slaves continue being slaves. If not, there's a chance they run away. Same for bases. If there isn't anyone at a base except slaves, they run off. This way, one or two slaves is feasible, but having more slaves requires some kind of investment. Let's just say regular mercs don't care enough about their jobs to stop them.
On the topic of running off, I think that if there are three or more slaves in your party with no other members (i.e. you and three armed slaves), there's a good chance they rebel. Having mercs/other players greatly reduces that chance, and having the slavemaster (we'll just assume he's the epitome of slaving badassery) completely eliminates that chance. The same would go for parties with other players who also have slavers. If the number of slaves is more than 2x the number of non-slaves, rebellion is possible. Anyone having a slavemaster at all would stop it.