I think it's the good ol' libertarian argument. The best decision a person can make can only be made by that person when they are well informed and not being held at any form of "gunpoint". A slave isn't free to invent, or build, or create. They are only free to do the labor they do not want to do, for someone who doesn't think that labor is important enough to pay people to do it. It leads to a massive waste in time, in labor, and in human beings. Society works best without slavery, it benefits the most without slavery, and it moves forward fastest without slavery. It is because of this, that modern society views slavery as absolutely abhorrent. Lets not also forget the fact that without slavery, without all that crap, we wouldn't have nearly as much racial tension, and society would be overall more equal in happy. It may have been good for the south in the sort term, but our former practice of slavery holds us back even to this day.