Personally, I kind of enjoy the 'looting' aspect of RPGs. Namely, if I'm going to invade some bandit camp then I want to be able to grab everything not nailed down or on fire, then pry up and extinguish the ones that are.
If I could 'enslave' some of my enemies and use them for pack mules to carry the loot back to sell (and then sell off the enslaved enemies) then I would probably do it. The only real limiters is how unsavory the people I'd have to make these deals with and the difficulty of moving the slaves around.
I'm actually reminded of the game
Space Pirates and Zombies where you can destroy enemy space ships, then scoop up the escape pods and recruit the survivors into your pirate crew and dump the ones who complain out the airlock (actually, its kind of expected since 'goons' are a resource right along the material used to build ships), there is a technology tree you can research to increase the percentage of those you recruit vs the ones airlocked but I tend to put more points into weapons and such.
So, if there was an RPG that allowed capturing your enemies as slaves (or maybe vampire cattle or whatever) then it would be interesting. I'm sure there would be plenty of of players using it to grab civilians and innocents and whatever (obvioulsy) but if it could be used alongside or instead of the 'kill everyone and loot their stuff' often seen in RPGs then it would be a nice change. Perhaps some of the 'good guys' ask you to go to a bandit camp and capture as many as possible to bring them back so they can serve their sentance.
The only real difference between grabbing crimminals to bring to prison and have them work on chain gangs, and selling innocent civilians into slavery is who is the 'buyer' and who your target is.
You could have similar 'enslaving' missions one run by the good guys to grab bandits and one run by slavers to grab whoever. They don't have to be 'good' or 'evil' just that one mission gives you reputation with the police and gets you hated by bandits while the other gets reputation with slavers and hated by civilians.
Though the act of keeping all these captured prisoners in line could lead to moral questions. Things like beating them up, using magic to mind control them, fitting them with explosive collars, lying to them, or whatever you have to do. Also, being sure to have your own team fitted with the weapons to keep them in line and make them think twice about trying to break free.