I don't have a link or anything, but I distinctly remember Yoshi-P answering a question involving being able to take our chocobo companions into dungeons with us, and he replied that while they had no plans to do that at the time, that he might consider it as a way to mitigate the issue of completing older content somewhere down the road.
Or, seeing as most content is rarely dynamic, it wouldn't be too hard for them to script NPCs for each role in a party and have them get added after a person or a group short 1-2 players has been waiting for a set amount of time, and only for content past a certain "age".
