Originally Posted by
Enkidoh
ACN having two Jobs failed because of an oversight on SE's part connected to the change to ARR - in that each class/Job's abilities were specifically controlled or guided by one stat, thus players would naturally pump most of their AP into that stat (like, MND for CNJ/WHM). This worked fine for a single Job, as each Job generally refined the role of it's based class and thus continued to make use of it's controlling stat.
ACN though, which was officially designated as a magic DPS, (and thus INT would be it's guiding stat), suddenly had a healer Job option in addition to it's DPS Job - thus those who put all their AP into INT and wanted to play as SCH found themselves stuck, as their healing spells (which required high MND) was neutered.
A band aid solution was found by reducing greatly the price for Keeper's Hymns (which allow the player to reset a class's spent AP), but it wasn't really fixing the root cause of the issue. Either way, it showed too many flaws with the system and thus SE abandoned trying to give other classes multiple Jobs.