http://www.bmwblog.com/2015/07/10/a-magnetic-braking-primer-bmw-i3-versus-tesla-70d/Magnetic braking only works through the wheels connected to the motor.
That means, if we rely on B mode for most of the regen, we're only using 2 front wheels for braking. Hmm, that does not sound as safe as having 4 wheels working together for braking. No wonder I hear tires squeal sometimes with B mode going down a steep slope, 'cause only 2 wheels are working and the rears are not helping.