There are only two upstream JSF code bases, Mojarra and MyFaces.  You would never use the Mojarra API with the MyFaces Impl.  In the past it has been theoretically possible, but anyone would be crazy to do it that way.

I would raise it as a feature request in the spec if I could come up with a really compelling argument for why it is needed.  I can't come up with anything better than "It's always been that way in the past" or "It just seems cleaner".