I'm not Mark, but since he's traveling and teaching this week, I'll take the liberty of replying.
These are different tools for different type of photography. For example, Mark and I are teaching a field workshop in Algonquin Park next week. The subject, along with landscapes, will be moose. This means long lenses. This also means stabilized lenses for when having to work hand-held. This also means light(ish) weight for hiking in the woods.
These are the types of shooting for which a technical camera with a high res back is totally inappropriate. Resolution and DR are secondary considerations, though one wants them to be the best possible given the other constraints. The D800 fits that bill. An Alpa with an IQ180 does not.
That's why some photographers own and use different tools.
Michael