The only advantages would be getting around the mythology token cap and.... well, that's it. You have enough space in your armory chest to have a unique set of gear for every class, so inventory isn't an issue. Also, FATEs are faster than running sidequests for levels, and after patch 2.1 they intend for dungeons to be on par as well, plus you get +50% xp if you already have a level 50 class. There's also many more issues that pop up when using multiple characters: First is that you won't be able to use cross-class abilities, you'll need to level another class up to 15 to get your job stone anyway, you'll need to level all your crafts up on ALL your characters if you want to be able to repair your own gear (Which is MUCH cheaper than paying the NPC), and you won't be able to get many achievements which require content done across multiple jobs.

Oh, and you can only have more than one character per world if you have a legacy subscription or the more expensive payment plan. The cheaper payment plan only allows one character per world.