If you are not heel-toeing, you are using the engine and transmission to slow down. It's just a matter of principal. The engine RPMs increase when you downshift. This happens because you either applied throttle and increased the rpms(ie Heel-toe) or the energy from the car's momentum sped up the engine. (in fact, anytime you're in gear not on the throttle/brakes the engine is slowing the car).
All that said, only one of our drivers is able to heel toe. The pedals are a mile apart. If I'm getting after it (max braking and quick downshift) the rear-end jumps on me during the downshift. Losing braking capability and therefore a slower lap time. If I'm gentle with my downshifts, I can avoid the hop, but I'm not attacking the corner as hard.
As far as our amateur series goes, heel-toe is pretty far down the list on what will make you "fast" but it certainly makes a difference for those quality drivers that can do it. And more importantly, it helps keep parts in their proper locations.