Why not? It's a beautiful hymn with a wonderful sentiment.

Like it or not, slave trading was legal in 1748. While we in the 21st century condemn slave trading as immoral, many of our 18th century forbears didn't share that opinion, and it would be another hundred-plus years before slavery was abolished altogether in the US. I don't think you can accurately judge an 18th century man by 21st century standards.

If you're truly concerned about slavery, visit the Polaris Project online, or any other organization that's against modern-day human trafficking.