Besides security concerns, we take advantage of Single Sign On. Our community server offers multiple services (nextcloud, gitlab, matrix, jitsi) and thanks to Oauth2 they can use same account for all of them. As a little plus, when you login to one service you are logged in all of them, and the same for logout.
I think the point if Oauth is useful or not is somewhat irrelevant. Is a fact that some users (for whatever reasons) have an Oauth based system and it seems the number will grow on time.
Of course I’m not asking anyone to implement nothing, I only say that it would be nice to have Oauth support 🙂