If they provide a blizz id (like Steam does), this would still be useful for other applications. Not to me, but for Steam for example – Steam is linked to our API. If we have their ID, we can have them make all types of fun API calls to get information about their account (beyond Steam) just because we have the ID. This is done via PlayFab (or GameSparks) – any type of BaaS
I bet there are other fan sites out there that do the same. But… Blizz logins are not necessarily POPULAR, so…