The main reason I would be interested in a high resolution body is the fact I gain 1.5xFL with good quality from the APS-C region.
Not only is a 1.5x crop capability useful, but also a 2x crop, and 3x crop, and even a 4x crop. A 3x crop of a 45mp image would be 5 mp, probably good enough for an A4 size print. A 4x crop would be only 2.8 mp, probably still good enough for an A4 size print, or at least 5" x 7" print, as well as an HDTV display,
depending on the nature of the subject and the quality of the lens used. If razor sharp eyelashes and sharp individual strands of hair are required, then 2.8 mp and 5 mp crops would not pass muster.
When I first visited this site, Canon's first DSLR, the 3 mp D30, was all the rage. Michael praised this camera for matching the resolution of 35mm film, at least on an A4 size print, but not quite on an A3 size print. We had to wait for the 6 mp D60 for that. The camera following the D60 was the 10D which had noticeably improved noise characteristics, but was still 6 mp.
Out of interest, I've compared DXOMark's results for that early Canon 10D, with the recent Nikon D850,
at the pixel level (ie. screen option).
Amazingly, at base ISO, the SNR at 18% level, for the smaller D850 pixel, is 2dB better than the 10D pixel, not significant, but better than nothing. The DR of the D850 pixel is very significantly higher, a full 2.5 stops better. Tonal range is also better for the D850 pixel. Color Sensitivity is at least marginally better, at base ISO. However, such improvements do not apply at higher ISOs, except for DR, which is still close to 2 stops better at all high ISOs.