I'm not sure of the methods used on the flight-deck, but to compare to modern T/Vs, the LCD component takes very little current, but the illuminating bulb is very much a ‘lifed' component.
The bulbs on a 50-60 inch screen used to cost $1,000, but now are around $350. One T/V shop in Essex does not leave the big Sonys on display because of the life of the bulb.
If there is a similarity in the method of displaying LCD information, it would be safe to assume that to under-run the back-light would give it more life.