The problem is that whatever is the quickest way to level is what people will gravitate to, and whatever they add or change to favor that is what everyone will do, forsaking other avenues. If they buffed chocobo exp and suddenly getting a chocobo party together and roaming the countryside fighting mobs in the open suddenly became the fastest way to level, that's what everyone would do, ignoring FATEs (unless it happened in their immediate vicinity) and dungeons. If it's discovered that you can clear a certain dungeon in mere minutes by putting together a group of 4 BLMs and speed nuking one specific dungeon, that's what you'll be expected to do, so I hope you enjoy BLM.

Point is, people will always gravitate towards what is easiest and most expedient. The game isn't lacking for options. You can run dungeons to get exp. You can do FATEs to get exp. You can put a chocobo party together to run around killing mobs for exp. You could even put a traditional PT together and go out into the wild and kill things. But people will, generally, only do what is fastest. If they do introduce something, if it's not the fastest means of gaining exp, people won't do it. And if it is, they'll stop doing dungeons and FATEs.