To play the Devil's Advocate here:
When you've been riding ICE bikes for DECADES you have a certain muscle memory that is not easy to recondition. I learned that when I rode my first Maxi-Scooter. As I approached a red traffic light my left hand automatically went for the "clutch" (in reality the BRAKE... please note the proper spelling... lever) and nearly bounced my face off the scooter's windshield. Remember how one applies the clutch; you squeeze it in QUICKLY and slowly let it back out. That is NOT the way you want to use a brake!
It took about a week of daily riding for me to finally GENTLY apply the rear brake instead of grabbing a fistful of it. This is especially dangerous if you're used to squeezing in the clutch to downshift for a turn, by the way.
I think Zero had the right idea by putting the rear brake where the majority of riders EXPECT it to be, and leave the left handlebar empty to keep us out of trouble. I see professional road racers are starting to experiment with a thumb-operated rear brake on the left lever. That idea I like, because we don't have a counter-muscle memory for that action. Then again, professional road racers never have to do panic stops, they just want to settle the suspension in preparation for a change of direction.