I'd delete any "2d" set in a heart-beat. It would be really annoying for gameplay (much like "invisible" sets like the Jrii). Just use some spheres and be done with it.
Well I would probably mix and match rectangles into a 3d shape with the texture applied - sort of like making a "paper-doll" model of a space ship etc. This would make it visible from all angles, but keeping a 2d-ness about the design.