I love Final Fantasy X-2's job system and whatever (totally reasonable) problems you have with this game I want you to love this job system too because FFX-2 has one of the most interesting and complex job systems ever made.
FFX-2 follows the adventures of Yuna, Rikku, and Paine. They are the game's only playable characters and this party doesn't change. Final Fantasy X had 7 player characters (and 1 temporary character) available for use while only allowing 3 in the active party.
In FFX they had a lot of characters and each character filled their own particular niche. Tidus was fast, Wakka could hit things in the air, Yuna could heal, and so on. FFX also allowed you to change who was in your active party mid fight. That you could easily change up the party mid fight meant that the developers could to push each character into a more specific and differentiated niches. If it turns out that Wakka is useless in the current fight you can just switch him out. If it turns out only Wakka can kill this particular enemy you can switch him in.
By letting the player swap characters in and out during a fight they expanded the range of usefulness each character could inhabit in each fight (ranging from essential to useless) without creating massive and unexpected changes in difficulty resulting from who you happen to have in your party.
As such the game was able to become a lot more about learning and adapting to your enemy mid fight. Most RPGs you have to have a pretty set plan going into a fight because you can't switch up your party or their equipment and so success rests upon your ability to make your pre-loaded plan work. If it didn't, then back with a better plan after you’ve died. FFX wanted to be about adaptation and learning. You didn't have to know what you were going to be fighting in order to be able to consistently implement a plan that worked against it.
FFX-2 only has 3 character but it does the exact same thing. Instead of letting the player switch up which characters they were using it let them switch up what jobs those characters had.
FFX-2 has ~20 jobs (depending on what you count as a job and what version of the game you are playing) and by having a lot of options available the game can push each option further apart mechanically and into their own special little niche. More jobs, more niches. And because of the mid fight adaptability that comes from letting them change jobs each job can be balanced based on situation usefulness and not on general viability.
In FFX everyone basically had their own job but they all existed on the same "Sphere Grid" and so were technically all able to learn all abilities. This makes each character much more adaptable (in an a-la cart sense) but it also means that every job has to share a basic mechanical vocabulary. Things like "Items" and "Attack" are things everyone needs to do becuase Lulu could, in theory, have a really strong attack even though her role in the party is that of a Black mage. So even if a character has a shitty attack that is basically pointless it still has to be there taking up space on the UI.
The job adaptability in FFX-2 though means we can drop things like basic attacks off of jobs like White/Black Mages. In FFX-2 if you are a mage and you want to "Attack" you have to change job which you easily can at any time. "Attack" on the various mage jobs is basically wasted UI space because of how weak those classes are physically.
Removing useless commands and generally cleaning up the UI is important especially since the game uses the ATB System and so battles happen in real-ish time. Scrolling through a hundred spells every time you want to attack is boring when the game waits patiently for you. When it doesn't wait, it is boring in a frustrating way.
You can't change to any job though, only certain ones. Each character equips a set of jobs and relationships between these jobs called a "Garment Grid".
This adds a new level to the complexity to the job customization because now you have to figure out which jobs you are going to want to use and which ones you are likely to need to switch between. Is the plan to start out Yuna as Thief and then once you have robbed the enemies blind change into a Black Mage or a Gunner (depending on the enemy's weakness) while keeping the option of White Mage open in case you need some extra healing? You'll want to be able to change back into a Thief if you run out of mana so that you can steal some MP which means you will want to keep White Mage and Back Mage adjacent to Thief. Not all Garment Grids will let you have all of those jobs next to each other though and so you will have to choose which transitions you are willing to have take an extra turn or two.
We aren't done yet though because there is even more to Garment Grids. Some grids have additional benefits you can unlock by going from one specific node to another in the middle of each fight. These upgrades can be stats, effects, spells, or access to another job's abilities regardless of what job you currently are. Different Grids give you a different number of nodes, different numbers of connections, and different effects for moving between jobs. The game is encouraging you to keep switching your job through out the fight, to change with the fight and adapt to what is currently happening. The game encourages arbitrary changes by giving you buffs for changing.
You unlock new abilities for each job by using that job in battle. You steal as a Thief and you get a bit of exp towards whatever Thief ability/skill you are currently trying to learn. Eventually, through enough use of that job you get enough exp and unlock that ability. Then you pick a new ability to learn and the cycle continues until you run out of abilities for that job. Each job only has so many abilities and as there are a lot of jobs each individual job has relatively low ceiling on how much exp is needed to master it (unlocked all of its abilities). You won't max out every job for every character by the end of the game, far from it, but you will likely max out a couple along the way. Once you have mastered a job any exp gained by using that job goes to waste. This is probably my biggest problem with this job system.
After having mastered a job using that job is wasteful as that exp does nothing. I really like it when a game encourages the player to diversify their playstyle. I think it is an important thing to do in order to keep the game feeling fresh, and keep the player learning and growing. But with this exp system the second a character reaches the zenith of a job then using that job becomes a last ditch option due to the waste inherent in using it after it has been mastered.
The player mastered that job though heavy use and probably used it heavily because they really liked using that job. Now they can't regularly use it because of the incentive structure on job advancement. The lost exp could of course be made up for by grinding but grinding as punishment for use of your favorite job seems like a jerk thing to do (also I’m against arbitrary grinding).
The goal should be to incentivise a diversity of playstyles, even force it a little bit, but don't punish the player for having a favorite playstyle. I agree that we shouldn’t let the player only use one playstyle, but this exp system feels like the game is saying you are done with any job you have mastered job and probably should never use it again.
I think that exp shouldn't go to waste but that begs the question: where should it go? We could let the player assign where it goes, "Send all the extra exp I got from using mastered classes to Dark Knight please." The player might use other jobs then because they just got new abilities in that job and what them to give them a whirl, but I don't think this is a strong enough inventive to get them to try other jobs. I think instead of letting them choose where that exp goes it should go into a pool that is used to buy random abilities from other classes. The player doesn't get to choose what they get, but they get something and so nothing is wasted (before any economists give me shit, yes, technically, efficency is wasted). This lets us keep the incentive of new abilities as a reason to play other jobs and means that if they really want to master a specific job or ability they have to use that job. They can still use their favorite classes without being wasteful but they still need to specifically use other classes in order to bring out that jobs full potential.
FFX-2 has problems, a lot of problems. I'm not going to try and deny that. But the job system is amazing. It has so many clever innovations and ideas that I don't see getting picked up or expanded upon. That makes me really sad because while I think this game's job system does a lot right it still has a lot of room to grow.