Fatigue exist in all game... most games you lose 25%-50% in about 30mins of game time when the "Rested Bonus" wears off. In FFXIV the rested bonus wears off slowly over about 50hrs (about how long it takes to get to 50% sp gain).

So in WoW 30mins to hit 50% sp loss or in ffxiv 50hrs until 50% sp loss. so there for the SP system in ffxiv is 150 times more rewarding then WoW's and most mmos rested bonus... and in FFXIV it even recharges when you are not playing that class.

Fatigue is negative in theory only, honestly fatigue is 100% negligible until you hit stage 5-6 surplus (~50% sp loss) which is longer then almost everyone play this game in 1 week. Stoping because of surplus means you have about 5hrs of playtime per class at the most a week.

This is all a Reward vs Punishment thing... they are exactly the same but one says you are being rewarded 200% sp the it drops to 100% and one says you are losing SP and started at 100%, but in the end they are both 50% sp loss.