Logging in with the full Stack Exchange OpenID URL is certainly possible, but not really something we want to encourage simply because it's confusing for anyone who's not exactly up on what OpenID is or how to find that URL. It is also a primary provider, and not one we're going to hide behind another click.
For the purpose of logging into our Q&A sites with StackId, email/password is the best way to do so:
I'm shopping around the idea of replacing the
firstname.lastname@example.org placeholder text with something like
your Stack Exchange OpenID email to see if that'll alleviate the confusion with existing users, although in all honesty, it seems like folks are adjusting fine to the new UI as time goes by.