its a feature that exists solely to make the game take longer, its not a good thing. If people plow through to cap, that's their choice. If they get bored at 50 with nothing to do, its their 'fault'.
By that definition levels are solely there to make the game take longer. So is EXP/SP curves. There is more to it than increasing the duration of the game, and while it is annoying (like repair, and mobs with really strong TP moves, and elemental resistance, and accuracy, and damage rating, and defense...) The list of things meant to balance the game is long, and is pretty much everything about the combat engine. The idea is to balance game play with combat. Since the gameplay is missing, all that is left is combat.
For some people that is ok, and they want more and more of it. Others just want the highest level game play, and nothing else matters, so they want to push to that, then experience all of it. Then you have the competitive people who's only desire is to be the first with the greatest, and have/be the greatest at whatever.
And it's the later two that tend to have issues with anything that limits their ability to play. For them fatigue is a bane, and for the last group, the new exp gain rate is also a bane. Since now, without the end-game worth playing, the ones who got to the top first just got to stand at the top of the hill with nothing worth proving, and nothing to set them apart from the people that are now making it there.
The irony is many of them have quit because they no longer can prove themselves. Others have started taking other jobs up, and well, at the end of the day, unless something is put into place soon so that they can feel a sense of accomplishment before they either feel crowded, or under-appreciated, they will continue to drop out of the game.
As for the more mild mannered players, most of us just want something to do. And while the exp gain helps in giving us a sense of purpose (to join those at the top), it still doesn't actually satisfy the need for SOMETHING.
So, my theory is, that at this point, the fatigue and the fast exp gain are just red-herrings for the fact that people just want something to do. While leveling is nice, the sense of accomplishment behind it is far more fleeting than having the next big baddy to defeat, or the next epic quest to complete. No amount of levels can fill that gap.
So, no matter how fast they make it, or how much they penalize us, the real problem isn't fatigue, or more exp, it's that there is nothing to do with the million + gil and exp we have.