Personally I have always thought of them as a hamfisted way of incorporating game play mechanics into lore.
As for the IC side of things, I don't think you NEED one to be any job. Yes, it helps having decades+ of someone elses experience at your disposal but nothing says that you can't be a "fresh" Warrior or a whatever. The first of the jobs had to start somewhere and nothing says you can't try to figure it out on your own. Your skills won't be as refined as those of the guy with a magic stone with several lifetimes of memories on it rammed up his rear end but you can still do it.
Except for White Mage. The elements will summon Odin to kill you if you even try it.
As for the IC side of things, I don't think you NEED one to be any job. Yes, it helps having decades+ of someone elses experience at your disposal but nothing says that you can't be a "fresh" Warrior or a whatever. The first of the jobs had to start somewhere and nothing says you can't try to figure it out on your own. Your skills won't be as refined as those of the guy with a magic stone with several lifetimes of memories on it rammed up his rear end but you can still do it.
Except for White Mage. The elements will summon Odin to kill you if you even try it.