I'm happy to change a server's content-type from application/json to application/vnd.my-adhoc-service+json, but that won't change the client's need to read my d

I'm happy to change a server's content-type from application/json to application/vnd.my-adhoc-service+json, but that won't change the client's need to read my documentation and play with the service before they can figure out how to use it. That's true even if I move the documentation out to an RFC. One real advantage I can see is to use the media type for versioning, but even that doesn't remove the need for a client to intervene if they want to upgrade their code to work with the newer API version. So it's not really clear to me that a custom media type is significantly more useful for the client, even if it is more 'proper'. When a constraint feels ceremonial, I become suspicious that it exists to drive purity-for-its-own-sake rather than a real benefit.