Having fixed classes also means you'll need to use all your characters and know how to use their special abilities. With fixed classes, you can't have a Juggernaut Tank character who deals 99,999 damage with his sword, takes 1 HP damage from every attack, can heal the party with Curaja and can cast Meteo and Bahamut, making the other characters pretty much useless. This not only makes your characters more individualistic (nobody is a carbon copy of one another), but it also adds challenge to the game.