People are putting way to much emphasis on the terms Job and Class. Jobs as they are planned are just quests to enhance a specific party role. At the core, you're still the class the you chose with some restrictions and some buffs. That's it.

This entire concept is not new either as SE has done this before with XII to a degree. Original XII had your characters fill whatever role they wanted and you could "cross-class" as many abilities as you wanted. FFXII: International Zodiac Job Edition forced you into specific roles throughout the game and you lost the ability to "cross-class". However, your job was several times more powerful than it could be in XII. SE combined this concept in one game.