I agree about the physical rendering size always being the same for a point, a point should be more general, but I still think there is some utility for having things defined in real-world units showing up as real-world units when measured on glass in the same z. An inch is an inch, and if it's too big for the display it should be clipped and if it's 100 feet away it should be unreadable. It would be useful for classes of devices--instead of special casing for resolutions, ratios, and densities, special case for desktops/laptops, phones, tablets, home projectors, conference projectors, etc.

But what's really missing is the explicit presence of a projection surface and the ability to adjust the factors that go into the calculation when projecting. It would be nice to have a unit that lets you say "I want this item to be as tall as the subjective inch if a person holds a ruler one foot away from them." That way no matter how close or far away I'm looking at the item (assuming the display is large enough--too small and too far will get it clipped, too few pixels will get it blocky), if I hold a ruler one foot away from me the item will be "as tall" as the ruler's inch mark. (Edit: I'm having fun trying to visualize a powerful enough 64x2 display built on a mountain. Where I grew up, there is just a single letter. From afar: http://upload.wikimedia.org/wikipedia/en/d/d9/Little_mountai... And from on top of it: http://2.bp.blogspot.com/-QeeaDcCiD4o/TexykEujKiI/AAAAAAAAHR...)