How do you justify the additional database load? We don’t know Twitch’s database design, but you have to consider that what you’re suggesting means an additional database lookup to retrieve user details to add to the response. As you already know there’s an endpoint for retrieving user data, so what this would be doing is duplicating that functionality and increasing load (albeit very small) on the root kraken endpoint.
Like I said though, I don’t know Twitch’s database design and maybe adding extra fields like these to the response could be negligible but personally I don’t believe there is a real use case for this as sure it might save you a few milliseconds by reducing the number of API calls you have to make but this process only happens the first time someone uses your app, so even if you have a thousand users that really doesn’t add up to much reason to bloat that endpoint for everyone when most apps (from my experience) will be using a token with a user_read scope and getting the user details anyway as being able to check that they have an verified email address is a handy little security feature.