Think of it this way: passwords are a more standard API than OpenID. Since passwords are the standard, they are "implemented" by all your clients. That cannot be said for OpenID.
I'm not suggesting "don't implement passwords" (which are first-party authentication) or "don't support password pasting" (which mainly supports ad hoc third-party authentication with a manual token exchange), but if you are actively choosing to support pasting (and, thus, third-party SSO with a clumsy UI), you should also strongly consider supporting third-party authentication with a decent UI.
The password manager solution is, if you'll allow me to strain your analogy a bit, "second-party SSO". That is, it's SSO that I, the user, manage however I prefer. Password pasting is an extensible API for achieving that.
But we don't disagree, folks should definitely implement things like OpenID.
