Most of the time, a drop is onto a fairly flat surface. IF the iPhone hits on an edge, the case supposedly cushions the edges, since it covers the edges, with both rigid plastic, and elastic rubber.
To my utter horror, I dropped my iPhone 4 onto a tiled floor yesterday. Thankfully it had a bumper on it, so there was no damage whatsoever. I'm already convinced of the bumper's worthiness!!! (And I think it looks good, too.)