RED, the makers of the beyond-4K cinema cameras, have a vested interest in making you want these higher resolutions. But they do explain it very well
One thing that any camera manufacturer whose products create images exceeding 4K has to do is convince you that ultra high resolution images are a good thing. RED has assumed this task admirably on their website, with a number of illustrated mini-tutorials showing how, even if you can't theoretically seen anything smaller, those extra pixels really do add to the quality.
You can find one such lesson here. It's all about how 2K video can be upscaled to 4K, but how this process will not add detail. It might be an acceptably compromise to see HD material on a 4K screen, but what you'll see here is that it's no substitute for the real thing.
We've always said this
We've always thought that the arguments about 4K pixels "being too small to see" are missing some very important points. Just as, with audio, there's stuff going on above the 20KHz or so that's recorded on a CD that can make a difference to the sound, more and smaller pixels, even if you can't see them, can definitely improve the quality of a picture. Aliasing is an example. If you look at a diagonal line on a screen, the further it is from 45 degrees to the vertical or horizontal, the more "steppy" it will seem. Aliasing can magnify the distortion that a pixel grid imposes on the world, and for reading small fonts and seeing tiny details, the more pixels the better.
We predict that most of the 4K naysayers will merge into the background as soon as 4K goes mainstream, and viewers see the real, tangible improvement for themselves.