I wouldn't use the word benign. Even those slaveowners that did could not have ignored the incredible brutality and subjugation that defined that institution. Thomas Jefferson hated the institution, but could not conceive of a southern - or of his personal - economic prosperity without it.
Other southerners believed it was paternalistic - according to them slaves' souls needed saving, and they would not be able to survive on their own, so slaveowners claimed to be merely "doing for the slave what they were unable to do for themselves".
In terms of profitability, I would have to say immensely so. Some studies have concluded that the average number of slaves over 236 years, accounting for working life span, average work hours and paid at minimum wage would come to a bill of nearly $15 trillion. While that has to be a ballpark figure given some of the guesswork that would have to come into such a model, the figure is undoubtedly in the trillions, and you can add that to the profit margin of the entire US economy during those years.
Of course slavery was not benign except in the view of the Southerners. They argued that slavery was benign because it was more humane than wage labor in the North. They argued that slaveowners took care of their slaves in ways that factory owners never would. They also said that blacks were inferior and needed to be cared for.
The question of profitability is much more complicated. Leading studies have concluded that slavery was profitable. However, you can argue that slavery was not as profitable as plantations with wage laborers would have been. This is because slaves worked so poorly (because they had no incentive to try) and because having slaves prevented owners from wanting to innovate with labor saving devices, for example.