Just think of some sprite from MvsC2 and other 2D fighting game. I think Capcom dev team on first MvsC3 had tried to stay some characters without switching their side like Spencer, but no use. Now they decided that they let them be anyway because of game mechanic's mirror visual model or something. Just like MvsC2 and other 2D fighting game.
There is nothing they can do about it... but they will wait until the new engine will be on consoles (PS4, Xbox720, etc). Maybe they have more advance than old consoles' technology.
I remember Street Fighter 3 when I met Gill. Gill can change to the side like in front of you, he can shoot ice. Behind you is the fire he can shoot. Strange enough, they did a good job with Gill, but I didn't think they can do with other characters, sadly.
--- "Get up!! Show me your true power!" Ryu's victory in SF3