The worry is that if systems like this become widespread enough then they'll simply be required just like phone numbers are. The website will pinky promise you

The worry is that if systems like this become widespread enough then they'll simply be required just like phone numbers are. The website will pinky promise you that they won't sell your data. Not to mention that now the government can just request access to all of your accounts on all of these services, because they know you had to use your ID.