Predetermined jobs makes more sense, but the option of changing jobs makes sense in some contexts. When you think about Cecil from FF4, he's a swordsman. That's what he has been trained to do. He doesn't know much about magic or barehanded combat. To make him able to effectively fight with his fists wouldn't make sense since he wasn't trained to do that.

In games with a job system, they usually explain it by means of crystals or something else that imparts the knowledge and undestanding of the jobs to the character. Typically the character lacks proficiency in a new job until they've done it for a while. In a game like FFT, it's not a matter of a character switching between swords and magic, it's more like the gamer decides to hire a different type of soldier to better accomodate what they're doing, and even in that case, the soldier still has to gain proficiency in the job before being great at it.

So I guess I'm saying that it would be lame if FF4 had changeable job classes because it wouldn't work with the story in the game.