One thing to remember about 4K in the "cinema" context (as opposed to TV flat panels) is although 4096x2160 results in a "square pixel" 19:1 aspect ratio it's pretty common for anamorphic lenses to be used to display super-widescreen/CinemaScope content without sacrificing vertical resolution; IE, instead of "throwing away" pixels they're all used, they're just not necessarily square anymore. (It's *possible* to fit extremely-high-end home theater projectors with lenses like this, but most people just fake it by using a zoom function to automatically best-fit the "active content area" of letterboxed material onto the available projection space.) Since the "normal" consumer television market almost entirely consists of panels instead of projection you don't really have the option to use adaptive optics or whatnot to keep from wasting pixels so it's inevitable that *any* shape is going to leave some people unhappy with how their content is displayed on it.
I don't think anyone would argue that 16:9 is the "perfect" shape but it was specifically chosen as a compromise between widescreen cinema and legacy 4:3 content (which technically includes movies made well into the 1970's). The TV industry *for now* seems to be concentrating their efforts behind pushing the 16:9 "UHD" standard in part for the same reason that Apple just quadrupled the pixel counts on their Retina iPads; they can sell it as being some sort of quantum leap in quality while letting it be 100% backwards compatible with its predecessor with just a hamfisted 2x scaleup of existing content. (Again, there have been a few 21:9 TVs marketed but in practice they've tended to bomb on the market. A big factor in this is that blue-ray is natively 1920x1080 and, just like DVD, it adds black bars to widescreen content. This forces a 21:9 TV to upscale and thus arguably lose picture quality compared to the same thing displayed on a taller-but-similar-width 16:9 set. Until there's a standard for ultra-widescreen content that doesn't require upscaling hacks this is a problem for *any* oddball-shaped panel.) Again, I'm not completely sold on this whole thing; 4K TVs do look awesome when you're face to face with them in the showroom and the TV is playing a native resolution demo, but in your living room it's really not going to make much difference and unless all you do is watch a small collection of movies and highly-compressed online content all it's going to be doing is upscaling 1080p-or-less feeds. And honestly I'm sick enough with the scaler on my 1080P set's (and it's 720p predecessor's, for that matter) attempts to lie to me already. Bleah.
