I have stolen this post from a poster on the Beta boards. I think it explains the reasoning and mind frame of SE oon the surplus issue nicely.
When designing an MMO, one of the things that needs to go in is some sort of limit on the possible progression of players. Developers need to control the rate at which players can approach the endgame, and other milestones along the way. Usually this is done with an XP curve.
An XP curve is really nothing more than a time sink. Developers decide how long they want certain segments of the game to take--how long they want certain level ranges to last, and ultimately how long it should take players to reach endgame. They then balance the amount of XP it is possible to gain in a given timeframe with the amount required to move beyond a certain a certain point, or level up.
XP gain may give the illusion of progression but that's all it is--an illusion. Gaining XP is not progressing a character. It does not impart any new strengths or abilities by itself. Levels are the real progression. Levels provide increased statistics and new abilities.
In reality, a game is designed and played from level to level, not from one experience point to the next. Being level 57 with 8000 XP towards level 58 doesn't let you do level 58 content any more than being level 57 woth 0 XP towards level 58 does. Being level 58 lets you do level 58 content. The XP required to get from level 57 to level 58 is merely a time sink. You spend enough time grinding it out, and you get to be level 58.
Experience point income is balanced against the total XP required for a level up in such a way as to have players spending a certain amount of time between level ups. This is true in any MMO, and Final Fantasy XIV is no different.
However, FFXIV puts a spin on the system. The developers for FFXIV are looking at it from the perspective that levels are the progression, not XP. XP is certainly not useless, but it is merely a means to an end. They also, like all developers, have in mind a rate at which they want players to reach certain level milestones.
For most MMOs, with a purely XP-based system, if the developers decide that levels should be gained at the rate of 1 per week, players have to grind away all week, making no real character progress, until they get that 1 level at the end of the week. THAT is the progress--regardless of whatever number is in the XP box, it is the 1 level that ultimately matters. And if you can't grind away all week, you don't get it. If it takes you three times as long to rack up the necessary game time, you get no progress on your character at all for three weeks.
The fatigue system is also a time sink, but functions a bit differently. It still works off of the concept that players should only attain a certain amount of progression per week, but it doles out that progression more quickly up front. This has two important effects. First, it means that even those who cannot play 6-8 hours a day every day can make some progress. Not as much overall, but some. Second, it means that after the week's progression on one class has been reached, players are free to mess around with others--a level of freedom and diversification that most games don't have.
Make no mistake; if SE decides that you should only gain 3 levels in a class per week, then that is all you will gain. Without fatigue--with a basic XP system--all your play time in that week would be devoted to a single class as XP requirements increase to compensate, and wanting to level something else would mean sacrificing progression in that one class.
With the fatigue system, instead of sacrificing XP time to do things that don't involve leveling your main class, you get to have all the XP you'd normally get for the week anyway, and then focus on other aspects of the game. Want to level another class? Go do it. Want to quest? Sure. How about crafting? Also an option. Or maybe you just want to explore. In other MMOs, all of these activities come at the cost of XP. That is not the case here. And if you really want to grind away at one class well, you can hop on it at the beginning of the week, and still have time to hit it again later in the week after surplus has cleared from not leveling it. Bonus!
Final Fantasy XIV was designed with a certain philosophy. The fatigue system is part of enacting that philosophy. It is no more limiting than a standard XP system, as progression is balanced around it. It offers more freedom than conventional systems, simply by allowing for different allocations of play time with no penalty. Can it stand some tweaking and balancing? Probably. But doing away with it entirely will cause a progression rebalance what will completely undermine any kind of horizontal character development, as well as force unnecessary choices between whether to XP a main class, or do anything else. And the ultimate gain on main class progression will be no different.