OK, maybe not where it is at, yet. But the Consumer Electronics Association (CEA) seems to feel it will be, or at least be common enough that it is worth working on a standard for signalling 21:9 content. The CEA is looking to update CEA 861, A DTV Profile for Uncompressed High-Speed Digital Interfaces, to include support for 21:9 signalling. Having a standard in place is important so that, as more vendors produce 21:9 displays, there is a unified way for content sources, like Blu-ray players, to communicate with the display to say “This is encoded for 21:9.” The display needs to be able to tell what the native resolution of the content is – such as 4:3, 16:9, or 21:9, to know how it should handle the display – letterboxing, pillarboxing, stretching, etc.
21:9 (which is more of a marketing term, it is really 64:27) approximates the 2.39 aspect ratio of many major films shot in anamorphic format. Most American films are 2.39 (anamorphic) or 1.85 (widescreen) these days. 16:9 is only 1.78, which requires letterboxing, horizontally compressing the image, or pan & scan when displaying a film, even at the lower aspect ratio. It is certainly better than the 1.33 aspect ration of our old, non-widescreen TVs, but 21:9 is the best ratio for viewing films.
However, as HDTV is shot for 16:9 displays, it is probably going to come down to what a given consumer watches the most. Someone who watches mostly TV is probably fine with a 16:9 display. The extra width would be wasted most of the time. Though you might use it for things like a social media scroll, PIP, etc., if you like to multitask. While consumers who watch mostly films may prefer 21:9 to get the full impact of an anamophic film. And with the CEA updating their standards, you’ll have the choice without worrying about compatibility.