I can stitch and pixel-bin my way to these pictures pretty easily, with $1000 worth of gear. So the use cases must be where that is not a feasible solution. Moving subjects? Need for quick turnaround? I dunno.
One obvious aspect is the usage of Leaf Shutter lenses that make it possible to shoot flexibly with strobes in the outdoors.
Another thing is the different look resulting from the larger physical sensor.
The camera's task here is to do the best possible job at enabling the working together of those Leaf shutter lenses and the back. I would think that the XF is best in class at doing that. Phaseone appears to have done a truly outstanding job here.
The resolution is in fact mostly of secondary importance. The value of the system would be 99% the same for a majority of its actual users even if the backs were only 20MP everything else being equal. But that would scare away some of the buyers who are looking for "the best" and "the best" must also be the highest resolution capturing device. Don't get me wrong, some users will benefit from the slightly higher resolution compared to the latest DSLRs but in the grand scheme of things, 80mp and 50mp is a tiny difference for most applications.
I do agree with you that for landscape, there are much cheaper ways to get to much higher levels of image quality through the usage of stitching when applicable. This is nothing new and the new camera/back doesn't change anything as far as this is concerned.
Cheers,
Bernard