Athiyk

Everyone that posted before me basically has the right idea. It's his style, it's his preference, it's just what he does, it's fairly new to the Kingdom Hearts series and adds a new sight to the battle system. In a practical manner, the reverse grip has its' benefits that the normal grip doesn't (flaws too, though). Of course, it also takes a decent bit of training, since not everyone can actually move their arm and wrist in the right ways. It shows his uniqueness as a character, and that he really has been training hard to become a Master. If you want a quick and easy answer? It just looks cool and they wanted to show he's definitely not Roxas. Or some such simplicity.