Whether an APS-C sensor is 10MP or 20MP, it has to be enlarged almost 14x its size for an 8x12 print. It's enlarged the same amount. Why should aberrations look worse?
Or I guess they could in very large prints, where the prints will often be much less than 200 DPI, but the viewing distance still has to be taken in to account*. For smaller prints, it doesn't matter because printers can only output at what, 300 DPI? And the naked eye can't tell the difference between 200 and 300 DPI (something along those lines).
In the vein of continued conversation, one of the differences between film and digital is that with film, you can tell rather easily the difference between a larger and smaller format even with small prints. Show me two 8x10 images, one taken with 35mm and one taken with 6x7--regardless of film--and I can unerringly point out the print from the larger format. Even if the smaller format is taken with higher-resolving equipment on a finer-grained film, the difference will be obvious and consistent.
With digital, it's nearly impossible to consistently tell the difference between formats until you start interpolating the smaller format significantly, and even then it becomes a dicey matter when the smaller format has a higher resolution.
This is basically the reason why some portrait professionals were able to use digital cameras for some purposes ten years ago and why at this point 99 percent of us can retire our film cameras entirely.
I would also point out that with photography, viewing distance is not really relevant in a discussion of print resolution (except in a discussion of the basics of what it means).
In practice, if you observe viewers of photography on exhibit (galleries, homes, et cetera), you see that during the course of their study of an interesting photograph, viewers will tend to move as close to it as physically possible. "Proper viewing distance" means nothing to viewers--at some point the average viewer is going to move to within reading distance if it's physically possible.
Viewers don't do that with other media. Nobody walks up to a movie screen or even a television set to see detail. Nobody climbs up on a billboard, nobody even gets close to a poster. But people will get close to a photograph because they expect a photograph to contain more detail than they can see at a distance.
This means that ideally--other factors being reasonably equal--that the photograph should be able to withstand scrutiny at reading distance as well as it,s expected to withstand scrutiny at any other distance.
It's true, though, that viewer expectations vary according to subject. For instance, viewers expect a portrait to resolve facial hair if it's to be considered a "sharp" portrait (if the portrait is clearly intended to be soft or blurry, obviously the viewer's expectation will be different). Viewers will get close enough to see if a portrait is that sharp--but no sharper. Portrait viewers are satisfied at the point that facial hair is sharp--they don't want to see skin flakes and hair mites. This is why some headshot portraits were successful even with low-resolving cameras--you can resolve facial hair in a headshot even with a 3 megapixel camera, and for a portrait that's all you have to resolve.
OTOH, viewers of landscapes expect the photograph to reveal more and more detail the closer they get to it. Their is no limit to how much detail they expect it to contain, given the opportunity to look closely at it. I once saw a guy whip out a loupe to study a landscape photograph at a gallery...but I figure he was a photographer. Considering this, the resolution requirement for landscape photography is effectively infinite.
