Well, if you think about it, slavery did have good outcomes. One, Economically, slavery was very beneficial to the landowners for whom they worked. Since they did not have to pay wages (although some did provide basic necessities) they enjoyed a larger bottom line (profit) This speaks to political advantages. Directly or indirectly, these profits were used to build the infrastructure of countries that had slavery. Two, it helped out the New World alot. I mean, slavers helped grew the population in the New World- 9.4 and 12 million arrived in the New World. The rich and wealthy people wouldn't give up anything and travel to the New World as well. And three, it founded the cultures, traditions, and the customs of slaves. Where would we be without jazz, folk tales, and many other famous things? 

Such there were tons of very bad things that happened towards slaves and changed them forever; but, you got to think deep in this topic. Since the concept of "slavery" is as old as man himself and has not been limited to any race or class of people throughout history, you might well say that, socially speaking, this practice is the foundation for even modern laborers. Those of us who work for someone other than ourselves contribute greatly to the wealth of only a small percentage of our population here in the US.

(Again, I'm not racism, just saying the facts).