Let you use a whole combo that's not native to your job? That'll require them to balance the combo on every single class. That's ridiculous and will inevitably lead to everyone's favorite update in the history of the world: http://dictionary.reference.com/browse/homogenization

(Not saying the current system is okay or acceptable at all. It feels like the bastard child of XI's sub-job system and early 1.0's armory vision)