> Why is 4:3 such a useful aspect ratio?
I don't know, but I agree with the question's implied premise (4:3's high utility).
It's a good question and I wish I knew the answer to it. I couldn't find any historical reference as to why 4:3 was originally chosen for televisions (the details behind the NTSC format are brilliant, but that's a separate topic). I don't feel anything like "boxed in" when computing on a 21" 1600x1200 CRT, and I don't want to give up vertical resolution for a widescreen of the same size. Lets speculate.
The closer the ratio is to square, the more usable area you have for the size of the device. If wider screens were better, why wouldn't we keep making them wider, why not 3:1 or 4:1 or 5:1 ratios? Maybe 3:4 is just a sweet spot for compromise between high area and our forward facing binocular vision. It's a mistake to even call them wider than conventional displays, as aspect ratio is independent of physical size. Have laptops really gotten wider, or have they gotten shorter? I think wider ratios are actually mis-marketed short-screens, with their prevalence reflecting cost (smaller area) in pushing HDTV sales, and not quality.
I know newspapers print in short columns for readability, as its easier to keep your place with short lines than with very wide ones, and computer screens were dominated by text long before graphics. Books too are mainly tall rather than wide ratios. Wider aspects are preferred for landscapes and juxtapositions of people in films, but whatever we gain in video game FOV we're losing in visible detail under our feet (and performance is lost to render peripheral objects you barely see, at increasingly skewed projection angles, versus more sky and ground in a taller ratio, which are virtually free performance-wise).
The bottom line is always useability. Do you really want to squeeze every vertical pixel out of an interface (browsers for instance), to deal with displays that are just too short? I sure don't, and I don't care to move a physical setup around when resizing display elements is sufficient. It may even just be tribalism or convention, but I know I like it. Long live 4:3!