I agree with what you're saying but the problem lies far more with other software not supporting @dir, @wrt and frankly plenty other tags more appropriately. Plex in particular is pretty bad with local metadata that isn't scraped. Media software does indeed rely far more on an ITUNMOVI tag for reading Director and Writer from MP4’s.
If it’s of any help, I made a script for The Movie Database that’ll create ITUNMOVI for you. It can also save a TMDB tag that you can then use as part of your media’s filenames to enable scraping: either method should get cast, directors, producers, writers and so much more showing up in your software.