well what I was taught, slipping the clutch is the same as friction zone. You disengage clutch a bit, so that just a fraction of motor's power is transferred to the wheel. Friction zone is just that gray zone between fully off and fully on. I personally use it, all the time when riding in heavy traffic at speeds where the bike is about to stall out. For switching between gears at speed I clutch very briefly. When hard on the throttle I up-shift without clutch, and for downshifting its just a quick stab with a quick blip, or smoother release with harder braking without blipping.
When I was just starting to learn slow maneuvers, way before I even knew what Gymkhana is, I got a DVD "Ride like a Pro" which teaches slow maneuvers that are used in Cop's rodeos. They ride pretty much all the time in the friction zone. However their courses are designed for much heavier bikes and slower speeds. With Gymkhana you are trying to haul ass and I personally think that by not touching the clutch, you are able to keep the power to the rear wheel all the time especially for those fast transitions and fast flicks into tight turns.