Maybe they can also give you faction points or whatever they are when they're reworked?
Also we should have an actual metzger slaver faction
Ahh right, i should have thought about slaver faction..
maybe +50
and ability to join ncr rangers 
With rep bonus from freeing slaves would you consider positive and negative effects, the positive being the one you have outlined here but the negative being increased hatred from slavers? unless you feel killing the slavers in encounters enough. Also what of the actual slaving? gain slaver rep for each one you catch dependent on type/difficulty and losing rep for rangers.
The only question then is would there be any particular benefits within the factions for high rep and can that just be abused i.e. you become ranger and have slaver friend, he slaves em and you free em.