Tying job-related loot to weakness from said jobs isn't a bad idea assuming the spread is enough to keep some mobs from going totally neglected because they're "not worth it" for it whatever reason. People killing Shen for the WHM melee body back in the day kind of springs to mind here (Obscure location, no other worthwhile drop). Anyway, making it so only those jobs could hurt or kill it, though, is pretty silly. It's just taking the job-isms from the player's hands and making the game do it with no compromise.