I'd like to add that this type of society seems especially applicable to goblins. I know that's obvious, but-since goblins already have trolls-I think it would add significantly, even profoundly, to goblin seiges, if goblins (and demons even more-so) were able to enslave *any* other humanoids, upto and including giants, ettins, minotaurs, etc. Maybe even any other creature. Gobbos might have general access to an "Enslave" skill that allowed them to yoke any and all creatures they encountered, and then harness them to their war-machine.
This would not only improve seiges, and make goblins a direct threat to anyone and everyone, but strongly differentiate goblins from the other races, as *THE* evil race.
Perhaps the children they stole, being raised into the society, would have their own loyalties and place in goblin society, making them better "tools" for the goblins to employ than their captured/dominated adult slaves.
Humans, to give a counter-example, might allow other species into their society, either as pets (dogs, cats), draft animals (horses, oxen), or in a "ghetto"-type environment, as second-class, but free, citizens. Humans might occasionally employ slavery as a means to an end, too, but not on the wide-ranging level as the gobbos, who would be out to forcibly integrate everyone, everything, and everywhere, into their society.
By the way, in order to not step on any toes: I did a search for a similar topic to post this idea in, and didn't find a more suitable one than this, but my search time is limited, so if there's a better place for it to appear, feel free to have it moved over there.