To me not turning it on seems like going out on a rowing boat and taking arm bands as your bouyancy aids... but not blowing them up first.
Its in the bag, its not turned on, you are becoming less sure of position. You are straight and level at 2300, 130kts. After another look at the map and the countryside you decide to turn on the GPS and because there's a few CBs around it takes 2 mins to locate itself.
From the start of that paragraph, to the end, has put you over half way across the Stansted zone.
Finally, its a machine. Unless its on and working properly, in front of your eyes, it is Sod's 43 law that it will fail at the time when you will be most strongly relying on it to work. If its working and stops working, you know that your dead reckoning is now your only nav option.
PB, I can understand leaving it off if you have a nice panel mount. And the Pilot III is a superb bit of kit for almost no money, if you can still find one.