Maybe we're talking about different things. If I (as a user) am sharing a video link from YouTube, in many (most?) cases I will already be logged in to YouTube

Maybe we're talking about different things. If I (as a user) am sharing a video link from YouTube, in many (most?) cases I will already be logged in to YouTube, and reusing that credential would be easier for me than creating a new account registration on the third party site. Edit: There's more: once you (as the service) have a YouTube account connected, you can offer a variety of additional services related to the YouTube Data API that a local account registration does not. (Also: depends on what you mean by "hassle".)