The fact that this measurement is an approximation (and a very good one in fact) doesn't make it "dumb". In fact, it's flat displays that are "dumb". An optimal display would be curved so that each pixel subtends a roughly equal visual angle, and it's only the fact that most displays subtend a relatively small visual angle that allows us to approximate this with flat displays.
A curved display (let's say, spherical) centered approximately at the "primary" observer's eyeballs whose pixel elements all subtend equal solid angles would (a) lead to weird unintuitive and hard-to-program-for locations in their most natural expressions the further you wandered from the center-horizontal or center-vertical row/column of pixels; (b) make it very hard to those whose eyeballs aren't smack-dab in the center of the sphere to make an intuitive mapping from their distorted view of the screen to something that makes sense.
I agree that a spherical display the size of a monitor or TV would be unweildy; however a head-mounted display with a large field of view would work best if it was curved.
