One of the problems is that it is inevitably hard to test things on a PTS without spoilers. To test Jobs at max level they need to do max level content which means unless they specifically spend resources on making content for the PTS, anyone accessing the PTS will be exposed to enfgame environments and by that almost certainly end game lore. PTS's inevitably mean spoilers. Maybe not quite as bad as WoW has them but it always has the same effect.

Honestly I don't like the idea.