Well releasing properly doesn't just mean it sorta releases; proper releasing meams that it the clutch disc can eventuatlly go to 0 rotation speed with no drag (will spin down by itself) with the engine revved up. Any small amount of drag between the disc and pressure plate or flywheel, or between the trannie input shaft and the pilot bearing/bushing, will keep the input shaft loosely spinning somewhat in sync with engine speed. The synchros then have more work to do to change the speed of the mass of the clutch disc and input shaft to the proper speed for gear mesh.