I learned in MSF that the front brake supplies 70% of the braking force when used in conjuction with the rear brake in a hard stop on the street. We actually did drills using front only, rear only (that was fun, locking her up like that), but front and rear combined had notably shorter stopping distances.
However, in all the panic-stop exercises we did, the bike was straight up and down. Even MSF teaches get the bike as upright as possible before applying the brakes, especially the rear.
With the bike leaned over pretty far, I will not even touch the rear brake. Even when I raced (back in the day, nearly 20 years ago), I never used the rear brake on the track. Under that kind of duress, unless you're jay or something, the tendency is to apply too much pressure to the rear brake ---> lockup