Obviously one cannot optimize for all conceivable viewing distances at the same time. Obviously one would like to optimize for a given viewing distance, because it is relatively easy to do (once one considers the Contrast Sensitivity limitations of human vision).
The fact is, photographers tend to want their prints to look good from any distance. That's presumably why some photographers spend tens of thousands of dollars on a 60mp MFDB.
The photograph is different from a painting in respect that the large painting really needs to be viewed from a good distance to be apreciated, especially impressionistic paintings.
Any photographic print, however large, will invite the photographer to inspect it from a close distance to see the resolution.
In the real world, the closer you get to any subject, the more detail you see. If your eyesight's not good enough at close distances, wear spectacles and you'll see more detail. Peer through a microscope and you'll see even more detail.
The photograph attempts to capture reality, or at least the reality we saw from the position we were when we took the shot. There's a certain fascination in being able to see more detail the closer you get, just as in the real world.
In fact, in the real world you may never get close enough to see the veins in the eyeball of your favourite movie star. A photograph may capture such detail so you can get close enough.
An impressionistic painting from close up, tends to look like crap, just a jumble of brush strokes. Likewise, a large advertising poster from across the highway might look terrific from a distance of 30 metres. Inspect it close up, and all you see are colored dots the size of small peas.
Perhaps the significant question here is, having optimised a print for viewing at a large distance, is it also possible to make it presentable at a close viewing distance without compromising those qualities appreciated at the larger distance.
Bart implies that it is, but I'd like to see some real-world photos demonstrating this. My feeling is that the exaggerated contrast of the print that makes it suitable for distance viewing, will tend to look like crap at close distances.