Personally with electronics, for me, they always seem to fail being switched on or off. Why push your luck switching these on or off in flight when not necessary. They are certainly useful in the terminal area!
It is like the captains who tell me 'no' when I ask for wing anti-ice on (A320)when I see ice on the icing indicator. I'm told no, on the 737 we had to wait until you could see ice on the wing. Yes, you are not flying a f****** 737 you are flying an A320 with wing anti-ice/de-ice and as per Airbus instructions put it on at visible icing on the indicator!