I use the B mode every day when I drive down 1200' of vertical, the engine may be turning, used as a pump as you say, to help keep the speed under control, but it's using zero fuel. My ScanGauge confirms this. My 2013 PlugIn display shows more regen when in B, then when just in D. What I try and do is use just enough brake pedal, while in B, (it's a steep hill, B won't provide all the braking force needed) to almost max out the dash display bar graph that shows how much regen you are making. To my knowledge, that leaves the friction brakes out of the equation, only when at the end of the bar graph display do the friction brakes do their thing.
Once at the bottom of the mountain, I have a nice flat commute into down, and I use cruise for that. As cruise won't work when in B (by design, but not sure why) I always take it out of B once on the level. I also drive a 54,000 lb crane truck, with a two speed Jake brake, and B mode is somewhat reminiscent of that, sort of!
Today's 13 miles commute into worK: 239 mpg (winter conditions, it's better in the summer), coming back (pulling the hill again) I only got 79 mpg.
