It's all balancing issues. The new SMG and Auto Mag are way too powerful, and dual wielding plasma pistols, spikers and plasma rifles isn't really a big deal. Also, on the whole "Miranda was dual wielding thing": In Halo 2 she was pretty much point blank in the Arbiters chest plate. If she braced her arms, she could fire 'em. In Halo 3, if she fired the shotgun or the Mag, she would have gotten her arm broken (Shotgun) or dislocated (Mag). Also, not everyone is ambidextrous, so firing with both hands could be hard for some people. On the canonical thing: Well, FireFight is the only place we've seen ODSTs use turrets, have the option to board Wraiths and pick up Gravity Hammers. It's not canonical because Johnson can be there, while he's on Delta Halo.
Hope that helped any lingering confused/surprised faces over the whole dual wielding thin. Peace.
~Hunter