tx_extra has the potential to make wallet based schemes simpler to implement.
This is really what Counterparty got right and Ethereum got wrong:
Not every node should have to check every custom token rule.
Complex rules only need to be checked by the wallets of the people that care about them.
https://twitter.com/spirobel/status/1625641515732336640
But that only works if there is the expectation that tx_extra wont get removed later on and the Monero community sees value in these kind of applications.
In the end these kind of applications cant be prevented either way, because it is possible to use steganography instead of tx_extra to embed arbitrary data into transactions. And there is nothing that can be done about it!
Possible applications are: A domain name system that links domains, usernames, and onion addresses together: https://twitter.com/spirobel/status/1619645634444931072
Simple cross chain swaps and auctions: https://twitter.com/spirobel/status/1621260531373072389
NFTs: https://twitter.com/AnonShopApp/status/1625163407195541504
and probably much more …
I posted this comment here before, but it was removed by reddit: https://www.reddit.com/r/Monero/comments/112efzm/removing_or_restricting_tx_extra_will_be/