Pretty much what Yurimi said about content invalidation, you can already see it in WP and AK or even the two story dungeons, there's no real reason to do them except to help out new people who need them for story purposes which is nice of you I guess, or just because they're faster and in the case of the Castrum and Prae if you really need the extra phils from the roulette and I guess WP for your relic materials if you can't find them cheap on the markets. It's why WoW just released a paid boost to level 90, because none of the older content really matters outside of vanity or after you're done with the story and everyone wants to instantly catch up to the latest expansion.
That's one of the main disadvantages of vertical progression and bland straightforward stats which could be easily solved by say, unbreaking the materia, stats and crafting systems and maybe allow that dungeon or even primal gear to be SB'd and have a higher chance of giving certain stats or ability enhancing materia, say you SB Pharos gear and have a chance of getting a materia that Enhances Cure or something like that, or you need materials from every dungeon to craft a good set of armor.