If the car keeps rolling over and moving the driver is likely to be less hurt than if he comes to a sudden stop against something solid from the same speed. Most rollovers tend to look spectacular though. A good roll cage is always a wise investment in cars like ours with flimsy roofs.
Studying the video, I'm not convinced that he hit a slick patch. Sounds like a missed gear change, downshift then caused the diff to lockup and the tail then got away from him. Exagerated lift-off-over-steer!
