Using a full 3D physics/graphics engine, I suspect, would make the creation of a 2D game much simpler. I think the focus is to remain 2D, for... speed? I don't know. Though, if I programmed a platformer today, from scratch, not being a programmer, I would use polygons and meshes of them to organize all information.
It would just be... 2.5D, or whatever you call it, for simplicity's sake. The playing field entirely 2D, game environment, collision stuff and appearance somewhere in between. SSB style.