1. Thomas, I do not assume they are examining the sharpness of the print. But, sharpness is one element of the print which will affect how they view the portion of the print they examine up close. When I look at large works, I am often drawn into the print and I want to examine small parts of the print. It is almost as if I am looking at an 8x10 crop of the larger work. I am drawn into the print for many reasons, sharpness alone is not the primary factor.
2. Ralph, Your response goes to the heart of the issue. Depth of field calculations are where this issue always arises. The assumption underlying the calculation is based on a proportional distance. But, this is where I believe there is a central disconnect for photographers who desire to work to a very high standard. They will do many things which others deem "obsessive." Nevertheless, in this area, they throw up their hands and say the DoF charts are good enough to follow. As long as there is enough DoF to make a sharp 8x10 enlargement, then they can go as big as they want because the viewing distance will change. But, for critical viewers, the viewing distance doesn't change.
The Jeff Foxworthly test:
If you have ever printed a step wedge or used a densitometer, you might be obsessive.
If you've ever tested the effect selenium toner has on paper D-max, you might be obsessive.
If you've ever use a spot meter, you might be obsessive.
If you are a member of APUG, you might be obsessive.
Shooting contact prints is the easiest way to satisfy the critical viewer. If it is sharp on the ground glass, it will be sharp on the print.
You can satisfy the critical viewer with enlargements. It takes a different approach to image making, however. DoF charts are set up for "I'm using this camera, what f/stop do I need to get everything in the frame acceptably sharp." I approach it the other way around. If I want to be able to make a sharp 20x24 inch print, I ask which camera I should use to accomplish that. Often it is simply a matter of shooting larger film, and making sure I have enough DoF to print big without sacrificing other aspects of the image. This is often as easy as choosing to shoot 8x10 instead of 4x5 or MF and stopping down from f/16 to f/22 or f/32 (being careful not to stop down too far which will degrade the image). Experience is a good teacher. I have enlarged numerous 8x10 negatives to 20x24 and they are sharp and have very good tone.