My personal experience has been that driving in B has no efficiency benefit. It essentially means that your engine will be engaged when slowing down resulting in lower MPG. So if you are driving down a ramp in B, you are saving your brake pads a bit, but its like driving a regular vehicle. In D, your engines will be turned off, batteries will still get charged but you may have to occasionally engage the brakes to prevent acceleration.

The only time I would drive in B is when either the (off)road conditions are not good or if I am driving downhill (at a steep lncline) for several miles where I wouldn't want to overuse my brakes. Hope this helped.