The heavier the bike and the lower teh center of gravity, the more rear brake can be used. Also, with a passenger, more rear brakes can be used for smoother stopping and less front end dive.
A good rule... the engineers aren't stupid, the size fo the brake is a good indicator of how hard it should be used. I beleive the rear brake surface area is less than 20% of the total braking surface.
When do I use only rear brake. At low speeds when trying to reduce the effects drivetrash lash at small throttle positions. I also mostly rear brake at low speed turns where the front brake can be grabby causing a loss of balance. Finally on loose surfaces I will favor the rear. For one reason, the rear tire is wider and therefore easier to control when sliding that the narrower front wheel.
For emergency stopping (I hate the term "panic stop". You never, ever panic on 2 wheels.) ... it's ALL front brake. Engine braking provides plenty of rear braking force... especailly on my BMW twin.
