To me, it seemed that including dual weilding in Halo 2 was a nifty feature at first, but ended up failing. This is mainly because Bungie implemented it in the wrong fashion.
Want proof? They had to release an autoupdate to make grenades and melees stronger and dual weilding weapons weaker.
The main consequence of dual weilding is that the weapons that can be dual weilded are just not powerful enough. Most people will take a Beam Rifle, BR, or shotty over an SMG, Plasma Rifle, or Needler. IMO when dual weilding the damage of the weapons should not be decreased. Weapons dont magically become weaker when you have two of them.
What does happen, however, is that accuracy is almost completely lost when you are firing two weapons at the same time in each hand. Go and try it out and u'll see

So the damage should remain the same, but accuracy should be cut down.
To make dual weildable weapons a viable option in combat by themselves Bungie should make it so that a player with an SMG can move/turn faster than a person with a Rocket Launcher.
Discuss.