In real life, not many weapons are for a set hand, so really its just a design thing guys. I think in part it allowed Nate to be able to shields designated as an off hand weapon only.

I have no proof, but I would not be surprised if gladiators were a bit more competent with their main hand weapons. I remember that being debated way back in the day when level 20 was the highest any gladiator had reached. (Things were slower back then. That was about 2 months of game play)