I don't want that garbage back

Well, it was a MAJOR burden for the game's design. Dual Wielding means a certain number of weapons have to be able to be used in this special mode, so that's going to be a lot of work for your animators and gameplay designers to begin with. Then in the actual game, the DW weapons have to be balanced so that a certain combination isn't overpowered or more powerful than the two-handed weapons. The end result of that complex tight-rope walking was a significant chunk of the weapons in Halo 2 and Halo 3 that didn't fit any niche role like all the other weapons in the game do. There were some exceptions to this. Dual SMGs and Dual Spikers were a potent combination in the right hands, as was spiker/SMG+Plasma Rifle combo, and the Plasma Pistol still fulfilled its role just as well as it had since Halo 1. But by and large, any of these weapons used alone (except, perhaps, for the Plasma Pistol) were extremely underpowered and not worth picking up unless you had a useful combination. And this isn't just a hinderance for the competitive side, this would affect the campaign and custom games. Because designing a game is difficult enough, but to code the game so that certain weapons work certain ways in one mode and different ways in another just puts a whole lot of pressure on developers that you don't need... especially if the end result is a bunch of weapons that are rarely used by players in the way developers expected them to. DW was a cool nerd-gasm inducing moment in the Halo 2 gameplay reveal, but it certainly didn't have the positive effect on gameplay it was supposed to, and I think Bungie made the right decision in removing the feature after Halo 3 and keeping it out all the way to the end of the franchise with Halo: Reach.