Not sure why you would even consider creating a character per class when we're given the ability to level them all in one. As you said, there are many very important pros to this system, more than you mentioned.

Your only Con is that you don't get to repeat quests per class and you only get a certain amount of leve allowances per hour. I find that a bit ridiculous.
First, as others have mentioned, you get an exp bonus when leveling a class lower than your highest leveled class.
Second, there are far more quests than you need to level a single class. It's quite easy to level your first class simply by doing the main story combined with FATEs, leves, hests, and hunting logs. Doing it in this manner allows you to have plenty of side quests available to you when leveling other classes. Not only that, but you of course again have the ability to do FATEs, leves, hests, and hunting logs when leveling that other class.
You'll be missing the story quests and that's it.