B mode is in place to act like a low gear from a typical vehicle. And the way you describe using it, sounds like you get it.
Anytime you press the brake pedal the vehicle goes into a regenerative braking mode, unless you are braking at very low speeds, such as parking lot speeds. I believe the lower limit of regenerative braking effect is about 7mph. Below 7pmh the system goes to friction braking.
B mode has the vehicle spin the ICE without inducing ignition. Thus causing the engine to turn into an air compressor. This is akin to a large trucks compression brake called a Jacobs brake. Anyway, the air resistance created by the pumping pistons gives you a significant amount of additional control while coasting or braking down a long descent. Do not worry about friction or engine use during B mode operations, the engine is fully lubricated and functions in the same physical manner as if the engine was running, except that it isn't burning any gas and it isn't running ignition.
I do not use B mode in regular driving, however it is very useful if you don't want to ride the brake down a hill or want additional braking assistance down a hill or long incline. B mode works at all speeds that I typically drive, I'm not a speed demon so I can't speak to 80+
mph. However if you are doing 80 on a long downhill you have other issues that must be dealt with before B mode will be of much use.