Slavery had it's place in Essos.

A child born to of meager means has little hope and would be fortunate to be trained as a dancer as opposed to a worker or soldier. Some people treated their slaves very nicely. A free woman of similar birth would be a prostitute. Someone would have to benefit from her training to put up the cost of her training in any trade. She would never be able to afford it on her own.

We look at this through the lens of "You're a special snow flake and can be anything you want to be." But the world doesn't operate based on that and Qarth is Dany's first dose of reality. It's not enough to think you're great, or even know you're great. You have to prove you are great.

But we need food service employees, sanitation workers, average people of average intelligence, the cogs that keep the wheels turning. It may not be as glamorous as being a creative or something but there is dignity in trades and I look at slavery in the context of Essos as a sort of union deal.

It's not nice, people should have choices, but what choices would they have if they were free? Look at Westeros, you're a poor commoner or a lord. Maybe you saved up enough to become a merchant or a minstrel, but even that requires some special opportunity that most will not be exposed to. In Westeros you have nothing but shitty choices if you're born poor and average. In Essos, you could be made into a great dancer, warrior, or tradesman. It all comes down to perspective at the end of the day.