Re using the rear brake under HARD braking conditions... think about that for a sec.
If you're braking so hard that you truly are at the limit, your rear tire is barely skimming the pavement.
(Just to avoid confusion, "braking hard" doesn't happen with the rear brake only... it means using all the braking power you have available, and that means the front.)
Watch the MotoGP guys and you'll see their rear tires are OFF THE GROUND under hard braking.
Link to original page on YouTube.
So:
What happens if you use your rear brake under those conditions?
If the rear wheel is off the ground, it will stop rotating right away if you're applying any significant force to the pedal.
If the rear wheel is just skimming, i.e. lightly loaded, it will lock far more easily than it will under normal cruising conditions.
A rear that loses traction (locks) and then regains traction = a highside.
This is why many track riders use very little if any rear brake. As mentioned above it's a tool for adjusting the bike's behavior, not slowing, and even then it's not used all the time.
Look at a supersport bike's brakes. There are two HUGE discs with multi-pot calipers up front. There's one disc about half the size with a single-pot caliper in back. That should tell you something: the vast majority of braking happens up front.