lurker's comment: How about this...You can enslave barbarians and keep them, but if you enslave a major civs unit, you destroy the slave. After all, barbarians could be considered as uncivilized, only fit for menial labor. If you don't get the benefit of your UU, then that would make it even more useless. I suppose playing the no unique units option would be the other possibility, that no one has UU's. Or would that hamstring the AI's too much?