I personally like dual wielding, and think it should be improved power wise, because while you're doing it, you can't grenade.
However, you could work in a 3rd weapon also.
As long as it's done in a way that's tested thoroughly. Brand new things tend to get buggy. Look at equipments and jumping, and mauler placement on default maps.
I do see a whole new factor it could add to MM and balance however. With 3 available weapons, you can cover more bases(long range, power weapon, tactical weapon, etc). It could really make game play grow stale.
IMO, not a bad idea per se, but not one I think Bungie could pull off successfully.