With 5.1-channel material, there is no need for centre channel steering control. Think about it: 3 channels, 3 speakers; what's there to steer? Each front channel simply gets routed to its respective speaker.

Well, there is a need with digital matrix modes: what if there are less speakers than chanels? In particular, what if there is no center channel speaker?

I suppose that it might be the case that the analog matrix modes allow fine-grained control wheras the digital modes do not. But, I can't see it being that hard to mix 1/2 of center into each of left and right if there is no center speaker, regardless of mode.