Having taken the advanced Police motorcycle course many moons ago (retired PO of 25+ years), motor officer for 2 years, part of out training, was emergency stoppping with the rear brake only. This was on a full dress Harley. You were taught NOT TO RELEASE THE BRAKE until slowed down considerably or you'd probably highside...
This was practiced in a grass medium (no curbs) doing approx 45MPH (in a very remote area with no other traffic).
As a side note, prior to ABS, (at least on Harleys) I got caught in an on-coming afternoon monsoon (normal during the summer in Florida). As I tried to get back to the station in a hurry and make a right hand turn, the back tire locked up (besides using the fronts). A bit of rear brake/ foot modulation put it under control on some slick highway.
Although most of the weight transfers to the front upon hard braking, the back still has some traction/input. Why would you not take advantage of it?
In my experiance, use both brakes and know the limits of your bike/brakes and how it behaves. Practice, practice, practice.