Depends really.

FF IV, for example would have been silly if I were able to transform Kane into the party Black Mage.

It depends mostly on the flavor of the game. FF V, while having unique and named PC characters, gets away with it because of how they draw powers from the crystals and what not.

I'll even give FF VIII a pass because, as non-sensical as the Junction system is in a flavor context (I'm not saying it's hard to use. The system itself is straight forward enough.) at least it works well enough to explain why anyone can do anything.

Ideally it would be an expanded system of Xenosaga 3 though. Multiple routes you can take with one of your characters that allow them different options, however all the routes are ones that make sense for the character in question.

There's also the Devil Survivor Route, which allows you to set the abilities, but the stats themselves are pre-done on all but your main. This keeps you to giving them set ups that make sense with their spreads, while also giving you some room for variety.

If we look at FF, the worst offenders of this are FF VII and XII. VII has a pretty big variety of characters, yet such minimal difference between them after the game starts picking up steam. XII is "Well, here is your board, pick whatever the smurf abilities you want. That Vierra who is suppose to be a master of all weapons? PFt, make her a staff wielding black mage."

Oddly enough though, I still like both games.


Edit: To finish, don't give me a variety of part options if the options I pick have little to no effect