Personally, I took an interest in seeing how efficiently I was operating the aircraft and I cannot see what is wrong with that.
Well, JW411, as BOAC has pointed out, the Data is completely meaningless, with atmospheric conditions, load, route, traffic, etc changing on every leg. It will give the Captain who loads less extra a more efficent fuel burn if all other factors would be the same as his plane is lighter.
So if any promotions or other boni are attached to such a competition, I find it utmost dangerous for flight safety.