The problem is that the 'door' is your password now - you have to remember all the different doors, or use a password manager to store them for you... But then why not just keep the actual passwords in the manager?
The door is just the identifier you use for retrieving a certain password. You can just use your username, or the e-mail address you used to sign up. Said identifier is not a password in that it's not secret. You can even write them down.