There is more to MF than merely the megapixels and potential print size.
The size of the pixel "wells", the type of sensor (eg ccd vs cmos), the way the information is processed
in the camera (even for raw), the type of lenses (eg leica S, Phase One, etc), all create a different "draw"
to the image than achieved from 35mm format. There is a three dimensionality of micro-contrast, texture,
feel that is visible from MF that is lacking from, for example, the D800E. Can you achieve a similar look from
the D800E-- perhaps with much post-processing, but it is still not the same -- and I have both a Leica S and a D800E.
I will be the first to also admit that the difference may be the lenses even more than the sensors, but that doesn't
change the conclusion only the explanation.
As to printer size, also remember that there are differences in the types and compositions of ink, the way it is
laid down (I was amazed to learn of the math involved in depositing the various colors of ink to create a "dot" -- even
the order in which the colors are "mixed". I have a Canon ipf8300 and the inks are quite different from the consumer
models and even other pro levels -- again, different "draws."
Also, it is my impression-- and that is what it is, not scientific fact -- that the files from the MF (eg my Leica S) upsize
better than those from MF (eg D800E). You can easily upsize the file to 40 inch prints that look fantastic (at least
on my Canon ipf8300).
There is certainly a reason to make such large prints-- besides perhaps what your customer requires. It is that certain images
need to be large, to engage the viewer, to allow the viewer to "walk into" or become part of the image. Some images lose
their impact when presented in a small size . (And, truthfully, vice versa as well.)
Hope this helps.