yes i would use that feature if it were available. I myself wouldnt use it to login but I would want my users empowered to be able to decide for themselves if they wished to use it or not.
I've looking into implementing SSO before in fact.
Not far enough to have anything useful. Was going to use SSO from github, gitlab, google, and LDAP to unify the services here. But was too much of an undertaking for the time i had.
depends entierly on how you configure it. Its not uncommon to rely on the external service to authenticate without needing a local password of any kind. Though that isnt necessarily the setup I'd recommend if you went with a SSO solution.
If a user doesnt setup their own password then yes, you'd always be relying on the external service. which also means that external service has some degree of power over your users.
Again this is why I said I wouldn't necceseraly suggest that configuration.