Is this a Troll? Is the OP seriously questioning if there is good reason to use the job/class system that makes FF unique and versatile compared to other MMOs? Its put in place so you can experience the full content of every class in the game without the silly nonsense of rolling another character.

I mean seriously. That's like asking if there is any good reason to not go take a dump in the woods when you have a perfectly good toilet to use!