Most Final Fantasy games that offer customizable party options APPEAR to be totally free, allowing you to choose whatever classes you want and roll with it. This has been a huge, consistant facade in the way the game actually plays, though.
For example, I am going to refer to [the original] FF3, FF5, and FF Dimensions, as they are some of the more prominant titles that always allow you to choose your party's classes.
In all 3 of them, you absolutely MUST have a White Mage at all times. MAYBE you can get around this by having 2 Red Mages that focus predominantly on healing, but for the most part, a White Mage is essential for sustained grinding and dungeon crawling, as well as keeping your party from getting completely wiped out by bosses. As a healer, the White Mage almost always has no attack power - and if they do [Aero, Holy], it often draws from the same valuable resource that your much more beneficial supportive power does, be it Mana or Mana Charges. Should you find yourself grinding for EXP or Job Points or whatever, a White Mage is a worthless waste of a valuble character slot, who exists solely to deal 12 damage per turn and patch up your meat-shields every 5 battles. Boring.
Secondly, in many Final Fantasy games, you will inevitably face off against enemies who are almost entirely resistant to standard attack damage. This means that your frontline fighters who've been carving through hordes of enemies at no cost are now worthless against these particular foes - and, as previously stated, you are also likely running a White Mage, who, again, ALSO cannot deal damage. This means that you MUST have at least one party member strictly dedicated to offensive magic... i.e., a Black Mage (or again, possibly 2 Red Mages to make up for their innate lack of -oomf-). Once again, we find that this mage fills only a niche, though necessary role as the "tide turner" - a character whose expensive abilities are only put to use when they absolutely have to be. Most time spent in Final Fantasy games are traversing dungeons and grinding, which you often encounter only weak or moderately powerful enemies that your primary fighters dispatch with little effort. Wasting 15 magic points to speed up the process is a waste, as you never really know (in your first play-through anyway) when a REAL threat might pop up, or just how far the nearest Town or Healing Spring is. Just like the White Mage, your Black Mage is going to spend a lot of his time knifing meager enemies and contributing fuck-all.
Then, with that out of the way, you're pretty much stuck with mostly-interchangable fighting classes (Knight, Dragoon, Ninja, Monk, etc) with your remaining 2 or 3 party slots, who, 99% of the time, are simply going to be using the same command in every fight, against every enemy - the "Attack" command. There are a few exceptions to this, but seldom do they pay off or are more tactically rewarding than simply pummeling an enemy until it dies.
"Niche" classes, such as Alchemists, Bards, Dancers, Scholars, etc just don't hold up. Ever. Even in scripted fights where their weird, unique abilities are often the intended focal point to counter a certain boss's traits just don't really work out - "Attack", "Attack", "Cure", and "[Black Magic Attack/Buff]" is always the "correct" way to play a lot of Final Fantasy games.
No, I don't like metagame any more than anyone else does, but in "customizable" Final Fantasy games, there are so few combinations that actually work, it's ridiculous.