I may not care who they are, but I do care how their identity maps to information in my system. Simple functionality such as favorites or saved preferences doesn't require any information about the user, other than them being able to identify themselves as someone who has previously used the site.
I'm not saying no one should ever use OAuth2, ever. Sometimes websites have to jump through weird hoops to get conversions. But as a web developer, it annoys me a lot.
I don't have strong opinions on OAuth or any other federated identity solution, I just think it's a specious claim to say that just because a system doesn't care about who a user is then that system doesn't care about the uniqueness of that user.
Choosing OAuth amounts to a very bad assumption that you'll never care about who your users are.