A system that punishes players for playing the game is bad. This kind of negative reinforcement is not good gameplay incentive. Switch fatigue to the rested bonus system, as it is essentially the same thing. The only difference is the negative connotation is gone, and once you hit "normal" XP gain, it won't go down any further than that. This would truly be a system that helps out casuals who don't play a lot, while leaving everyone else alone to do as they please.

For those of you who think the fatigue system is good because it encourages you to level other classes, there are better ways to incentivize that behavior other than the stupid fatigue system.

For one thing, make horizontal leveling have more of a benefit. Make the rewards for ranking up multiple classes have a larger benefit than there is now. Allow me to purchase more cross class affinity. Let each class have a quest that allows your character to have 200 more maximum TP, or 1 more accessory slot point, or something that allows you to preserve your strengths in one class and carry them over to another.

TLDR: The fatigue system as it is currently implemented is annoying to most players and should be changed to a rested bonus system.