I teach the MSF (Canadian Equivalent), I race, and I ride street.
Dragging the rear brake is done in the "friction zone." It allows power to be sent to the rear wheel, making your bike steady, yet not go too fast. The friction zone is done at SLOW speeds (under 8mph, basically). Dragging your brake in a corner with the clutch all the way out is stupid. You will just spend more money on brakes and fuel.
Turning a corner with no power to the rear wheel makes the bike unsteady. That doesn't matter where you do it. Track at 80mph or street at 10mph in an intersection. Never squeeze the clutch at higher speeds in a corner.
Ride your bike at idle with the clutch out and you are doing over 8mph. Wanna do 4 mph? Ride the clutch. Wanna control the speed? Use the rear brake. That puts power to the wheel, but controls speed.
Sportbikes, by their weight distribution are super hard to control at low speeds and that's why most people are clutch out or clutch in and never in the middle, but the middle is needed, sometimes.
Watch vids of Daytona bike week and Sturgis rally and you can see the complete lack of control people have because they don't know how to "slip" the clutch on their cruisers.
The point of the MSF techniques are not to teach one to ride at 60 mph. ANYONE can sit on a bike on the highway at 60 mph. The bike doesn't need you at that speed. You can be replaced with a rusty throttle cable.
The MAJORITY of motorcycle insurance claims are from parking lot and fuel station drops and that's why the insurance companies give a discount for having the MSF course to new licensees.
Whether it's in a parking lot, highway, or track, people biff it because they aren't smooth in their inputs.