That old law was probably written before syncromesh transmissions and good brakes to prevent runaway down hill cars and more likely trucks. Some of the steeper Highway hills I have gone down I end up leaving it in gear to control the speed and take advantage of the fuel cutoff. Holding the clutch in at high speeds in gear with the engine idling can do some serious motor mount damage if your foot slips off the clutch but I sometimes hold it in if I need to pick up a little more speed before the bottom or to make a light.