Although I could see some fringe benefit to being able to do this, you can already pretty much have the same effect by not equipping new gear. Gear matters a LOT more than character level when determining the power of a character, and you already get scaled down for dungeons and raids so overleveling isn't a problem.

Doing things like completing the hunt or craft log would be even more not worth doing if you had your exp locked and they gave literally no exp.