Let's say the system is like this
I'm a PUG 50 i have level all class to 30 and unlock all the jobs pretty much now i have all max Jobs to 50 by just easy putting on PLD on my pug i now become PLD and can tank. Oh i need to heal? i switch to white mage i need to nuke i switch to black mage. You pretty much got all max jobs by only level up one class 50 and the rest 30. This would be a big exploit in the game and a game breaker at that. So your idea make no sense and it would just mess up the game.
Cairdeas you still thinking of a hybrid system how the hell do you know if jobs define a wep or not? if am a Conj i unlock white mage and only Conj can use white mage or black mage that what make sense not a dam Pug using a hand to hand wep. We trying to give role to class not give everyone the chance to pick any role no each class would have a few role to be not everyone can be any role.
we pretty much be back where we start allowing everyone to be anyone how they want and what wep -.- no that make no sense.