Well actually...pretty much every FF game that didn't have a job system or something similar, like the sphere grid or license board, the characters pretty much had a set role. Cecil was a physical powerhouse with good defense and some white magic. Rosa was a white magic user that could use a bow and arrows or staves. The list goes on. You can fit each character from those games into a specific role even though they had multiple angles to them. Like, you're not going to use Cecil as your main healer, even though he has white magic, because Cure 2 just isn't powerful enough.
My take on this is the FF MMOs wanted to use the job system featured in many of its games so that people could experience every role on one character. If they wanted you to only be a Warrior or a Thief for the entirety of the game, it would have originally been designed that way. And I'm sure many people still would have played it, but this was one of the unique things FF brought to the genre.