[Brightcolours]

DOF is definitely not the main reason. A photographer like Ming Thein for instance, choses MF over FF because of the pixel quality and resolution he gets from MF. Higher DR, better tonal response, etc. and also higher resolution. Although a Canon 5DS might have a 50MP sensor, few (if any?) lenses can resolve it, especially across the frame.

The 50MP MF Fuji will very likely have lenses taking advantage of the body's resolution. In terms of resolution the Canon and Fuji won't be even close.

The test images on dpreview already show that the 5DSR does fine resolution wise, compared to the Pentax for instance.