We just don't know a lot about how cognitive resources are utilized. Long distance runners know this. Athletes know this. The whole concept of "second wind". Where they find the strength to better their game using way less resources -- after they have been tired. Some type of cognitive resource depletion gives people even more energy and motivation.
While I agree that things should be made simpler and we shouldn't over-gamify things, I don't think we should make decisions with the cake / fruits question in mind. That just provides a framework to dumb things down. We will never enable the users to hit their second wind if they never get tasks that make them crave cakes.
I guess my point is: simplicity is good. Simplicity to the point of dumbness is not.