A good reason to use "B" only for maintaining speed on long downgrades. Regenerative braking (beyond the very mild amount used to simulate coasting with an AT) does turn on the brake lights.

About the time the high mounted brake lights were mandated, the local buses had an experiment with a three light system. Red for brakes, Yellow when coasting (engine braking), and Green for power. I really liked it but I guess it wasn't deemed effective because they dropped them.