We don't own slaves, and none of us are slaves. This is an unambiguous good.

But when you look back at history, slavery was pretty much universal. Sometimes subsets of slaves became incredibly weird and powerful, like Janissaries and Mamelukes, but some institution of bound labor was something everyone had as part of their cultural toolkit. Many variations existed, from house slaves that were able to behave more like servants, to field slaves that might work side by side with a small farmer--even Ulysses S. Grant did this, in his farming days.

And brutal large enterprises, like the latifundia of late Republican Rome that Tiberius Gracchus used as justification for his reforms or Athenian silver mines, used slaves up without mercy.

Anything that benefits us can be found to have a moral justification. If we don't have property we define property as theft. Once we have property to defend, our attitudes mysteriously change. Most of us like comfort, security, and pleasure, and become enormously resentful if some change threatens this arrangement. And in history, comfort, security, and pleasure were rare enough to be worth fighting for savagely, and accepting the enslavement of others to achieve.

And now? Who needs hewers of wood and drawers of water to be comfortable? Our thermostat turns on the furnace, and water comes right out the tap at the right temperature. Vacuum cleaners clean better than a brigade of maids with feather dusters and brooms, cars take us places faster and more comfortably than a sedan chair or coach, washing machines keep our cotton and synthetic clothes cleaner and more comfortable to wear than any handwashing of linen and wool.

And owned human beings require food, lodging, and care. They get sick, they get old, they get violent, they try to run away. They're high maintenance. Russian nobles often had serf orchestras. Much cheaper to buy some speakers and download some mp3s.

SF novels where slavery returns in some form seem to think that oppression is the point. They contend that people own slaves to express their power. Pushing other people around can be fun and emotionally satisfying for a certain type of individual, but this is quite secondary to the comfort and service they provide. Slaves that can't make your life physically more comfortable are a too-expensive luxury.

So if we look back at history and congratulate ourselves for our relative virtue, we haven't really earned it.  Really, to consider ourselves virtuous, we should all be saints compared to people from past centuries. But I doubt that ethics classes indicate that the most powerful force for good behavior toward others is the one that brings us microwave popcorn and HD TVs.