The A7r user is gonna want to print some big pictures to gain an advantage.
This is most common confusion and misunderstanding repeated all over different forums from person to person and this resulted that this is taken as granted by majority of the of people who are arguing about value of high MP sensors
In fact this is totally wrong.
Downscaling high MP image to the lowest resolution camera image in comparison and viewing results will never demonstrate how high MP image will look on real print. Viewing this results on the PC monitor is the same.
This is actually what many people are complaining about DXO sensors comparisons when they are normalizing images to 8mb. At these days it would be better to normalize images to 24mpx which is de facto common reference point now.
Now coming to high MP image printing.
Here are A4 and A3 printouts resolutions at fine 720dpi and superfine 1440dpi
A4 8.3 x 11.7 inches
Pixel size for 720 dpi = 11.7x720 x8.3 x720 equal to 8424 x 5976 = 50.34 MP
Pixel size for 1440 dpi = 11.7x1440 x8.3 x1440 equal to 16848 x11952 = 201.367 MP
A3 11.7 x 16.5 inches
Pixel size for 720 dpi = 11.7x720 x 16.5 x720 equal to 8424 x 11880 = 100.077 MP
Pixel size for 1440 dpi = 11.7x1440 x 16.5 x1440 equal to 16848 x 23760 =400.3 MP
So from above it is clear that in fact even most Buyer sensor MF cameras can not over perform standard quality 720dpi A4 printing resolution. On printout every dot is compete pixel . For Buyer sensor effective resolution is about 60-80% of actual pixel count. So only Phase One IQ180 image could match in resolution to fine quality 720 dpi A4 printing resolution and far below of fine quality 1440 dpi A4 print let alone A3 printouts.
So difference on IQ and resolution between 24mp and 36mp will be clearly seen even on A4 printout.
Even on small screens difference in resolution is very noticeable.
This is why people want to have retina displays on their smartphones and now new trend is 4K screens on the smartphones , let alone TV set.
As for big prints at very high resolution - they give you feeling of the real reality. From far distance you see overall image, when you get closer and closer you see more and more details - the same as in real life. On big landscape print from 400mps image you get very close and you would be able see even bee on the flower and flower petals texture and fur on the bee belly. And you will fill this as it is real reality