Nobody except me, of course. I'd be thrilled if I could open up a JPEG in Emacs and e.g. view/edit the EXIF metadata in a text buffer (and/or pop up a graphics buffer for editing the image itself). Similarly, I'd be thrilled if Emacs was able to parse MessagePack data and let me view/edit it in a buffer. Both of these things are theoretically possible, but to my knowledge nobody's actually done them yet.
Granted, calling Emacs "just" a text editor is pretty stretchy, but still.
JSON vs. Bencode, JSON can get you the data good enough a lot faster, while a bencode parser is easier to write. My feeling is that most formats with schemas usually fail somewhere in the details anyways. Honestly I think I want too much from them; e.g. the YANG Data model language is a kitchen sink but lacks a lot.
Nobody says "what a shame I can't open that jpeg image in a text editor".
Everyone understands that an image is an image, and there are tools for dealing with images, like photoshop.
The same could be said for data interchange formats like json.