In fact, Ian, it's not just MIDI for the extensions. It's MIDI ONLY for several important chord types (dim's aug's sus4's etc.) so the track could be constantly switching in and out depending on whether you use those chords or not.

This is disappointing, especially when you consider that, if these things stream from the HD as we have been told (rather than have to be loaded in RAM), what difference does it make it there are three chords (maj, min and 7th) recorded, or a dozen different kinds? Other than of course, the complexity in MAKING the loops. But the implementation of them shouldn't be any harder, you would think...