But it requires you to have an account with some other party, such as Facebook or Google. What if you don't want that? Suddenly this service that you do want to use becomes unusable.
And it also tells Facebook / Google etc what other sites you use, which is none of their business (not that they won't grab that info because of like buttons, analytics, embedded fonts or hosted JS but that's another matter).
Can't have your cake and eat it too. If you complain that you don't want to have to log into sites, it makes less sense to complain when there are ways to have unified logins so that's not necessary anymore.
And it also tells Facebook / Google etc what other sites you use, which is none of their business (not that they won't grab that info because of like buttons, analytics, embedded fonts or hosted JS but that's another matter).