AMcGuinn has a nice post on slavery, in which he comes to conclusions very similar to my own:
The true argument for slavery is this: that those who are not able to support themselves are necessarily slaves, and abolition ultimately amounts to an exercise in creative linguistics.
. . .
Back to those conditions: ideally, all those capable of freedom would be free, and the incapable should be given the best chance of becoming both capable and free. But there needs to be some compromise here. The welfare state is geared to the capable but unfortunate, is grossly unsuitable for the most incapable, while at the same time dragging far too many of the marginally capable down into dependency. There seems ample room to improve on it with a system of humane serfdom under which a serf is subject to a lord who his responsible for his support and humane treatment.