Tell you when I use B, apart from down hills and town driving.
You are bowling along the motorway, in D, at 65 mph, and you want the next exit. If you wanted to leave at 50 mph, the BMW/AUDI in your rear view mirror, would be pushing you along, or making an overtake manoeuvre, even though he will have to slow down or stop, in 70 yards, at the oncoming roundabout.
So, I exit at 60mph, and leave my slowing down, until much later. Then for the last 40 yards or so, I pull the lever back into B, and have the pleasure of seeing my battery meter, going from 5 notches, to 8. And all without giving the game away with brake lights!