When I rode a motorcycle I let the clutch out fairly slowly, not as slow as the "feathering" when setting off though. However most quads are "semi automatic" which, in fact, means the foot pedal works both clutch and gearchange, riders just bang them through the gears and they don't seem to mind the rapid letting out of the clutch that the foot pedal gives. On electric shift Hondas, an electric motor moves the "pedal" and once the motor switches off, spring pressure bangs the clutch back in engagement as fast as it can. So, in short, it doesn't seem to matter all that much.