Knowing SE and their past explanations about inventory issues, I think that they did that change due to the chocobo saddlebag, and not just for aesthetical reasons. Pre-4.2 we could access at once the normal inventory, the armory, and one retainer. Now you would have to add there all the slots from the saddlebags, which means that they need to potentially load more items at the same time, which implies an increased server load (which is what they always complain about when talking about giving us more inventory space, not about the actual space that it would take on the database). So they probably reduced the amount of slots that can be seen at once to accomodate for those new ones.

I really hate the change too and I've been tired of the "server load" excuse for a long time already, but I guess that that's more than likely the explanation that they're going to give (if they ever do it).