Prints are certainly the area that gains the most, especially if you print really large.
You have to print very large to see any gain at all today. In print at 24" I would say there is no real difference between the 5D3, A7, A7R, or D800. You might occasionally spot some small difference with your nose on the print, but most subject matter simply won't show it.
We greatly overestimate our ability to discern detail at a normal viewing distance, even those of us with 20/15 and 20/10 vision. It's really humbling to produce two big prints (19" or 24"), same subject, taken with a modern and an older sensor (say 10 vs. 18 MP), ask people if there's any difference, and have 9/10 say no
I'm not against more MP, I'm just realistic about their impact. A 24 >> 50 MP jump will probably be visible in a 36" print or larger. But 24 >> 36 or 36 >> 50? Meh. The magic number seems to be a 50% gain on each axis.
For one, it could pretty much eliminate the reach gap between APS-C parts and FF, assuming you could maintain a high frame rate (and we know that's possible...Canon achieved 9.5fps at 120mp.) You could crop any part of a 50mp frame, and have the same kind of reach as a 20-24mp APS-C camera.
Second, even if you aren't printing, downsampling 50mp means your images sharpen right up without any actual sharpening. Just the act of averaging more information into less space improves your IQ. You could get away with less NR and no sharpening at all when scaling for wallpaper and web sizes.
True, but not much of an issue for the cameras we're talking about.
Third, 4k screens are going to become more common, and eventually common place, within the next few years. At native size (unscaled...currently some browsers scale images along with text DPI), to keep images looking like 4x6, 5x7, and 8x10 prints on screen, they will need a lot more pixels than they currently do. That enhances the second point...starting with more pixels, you can downsample to those relative sizes for native display on a high DPI or 4k screen and still have the benefits of increased sharpness/lower noise.
A fair point though, again, I don't think it matters too much for this class of cameras.