Dual wielding just presents balancing issues, without adding anything meaningful to the game. Any dual wielded weapon can just be thought of as another entirely new weapon. Bungie could just come out with a new weapon that has the same characteristics as the dual wielded weapon. Now if these can be mixed and matched, like in Halo, the balance of every combination needs to be tested. With single wielded weapons, you can still have the same variations in firing and accuracy that you can have with dual wielded weapons, and you don't have to spend extra time and money coding a dual wield feature.