In what real-world situation does the 5DIII have insufficient image quality for the job but the D800 does have sufficient IQ? That is, when would you put down the 5DIII and pick up the D800 instead of a medium format kit?
Currently I primarily use a 7D plus assorted telezooms for wildlife. Lack of reach at a reasonable quality is a limiting factor, but at the same time, I need to be ready to adapt to changing situations. Typically I'd go out with two bodies, one tele zoom, one wide zoom. I would love a high MP density FF body to use. 36MP full frame would be an acceptable minimum, although a Canon parallel would be 46MP (make the 18MP APS-C sensor bigger). Basically this would give me the same effective quality and reach as the 7D by cropping on the long end of zoom. When I don't need to crop, I can make use of the increased resolution. Also, by being a bigger sensor, the zoom just got extended 1.6x on the wide end. This would increase my flexibility significantly as I look towards longer focal length solutions. Yes, I'd trade off the fps for the increased resolution. For obvious reasons medium format is the wrong tool for this job.
e.g. using the 100-400L as an example.
On 7D: 160-640mm equivalent at 18MP
On hypothetical 46MP full frame sensor: 100-400mm at 46MP, 400-640mm effective at >18MP by cropping.
Increased resolution over most of the range, while maintaining the total reach, and more effective zoom range compared to 7D.