Well, Toontrack sells a lot of MIDI patterns for both EZKeys and EZDrummer, the expressivity coming from being recorded by a human being.
PG Music does something similar with the MIDI supertracks.
But the developement has been centered on producing Real Tracks, as that's what people want; having real musicians playing what you tell'em to following a "pattern", called "style".
I don't see this changing anytime soon.
But I can see an enormous market for acquiring MIDI styles played by human beings, just as solos for melody creation. Eight and/or sexteen bars, that can be easily put together in RB to further editing would be a base to build from.
To support this, RB should be entirely re-written; as is, it unfortunately is much too clumsy and cumbersome to work with. The paradigm itself should be revised.
For all these reasons I don't see it happening any time soon either. Imagine the whish list petition: "change everything in Real Band".

HTH,