John Norman makes clear that there is a paradox within slavery. As you have said, slaves are, by nature, subservient, submissive, and socially inferior; in these ways, they are indeed lowly. However, they are also radient women-- intensely passionate, keenly intelligent, astonishingly beautiful, richly nuanced, vibrant, whole. In that sense, there is nothing lowly about them. Slaves are glorious creatures. Being Tuchuks, we feel our slaves are even more so the ideal of the collared woman.
I do believe it was this latter side of slavery to which Master Phoenix and kadi were referring.