If they are(become) a playable faction, who would they enslave? I would hate life if i got rolled up by slavers and i have to start over because im stuck make big rocks into small rocks all day for no caps... maybe if you do, you lose all your gear and use your "Sneak" skill to get away? But at that point, youd be hunted for being a run-a-way the way slavers do... On the other hand, say you join a slaver band for some quick caps, who would you enslave? Just NPCs? If you enslave a specific NPC would he/she dissapear for everyone else who plays?
The best answer to these questions has got to be "Dont become a slave" or "Dont join the slavers"... but thats the great thing about Fallout... you can do what you damn well please, as long as you can live with yourself at the end of the day...