I'm a little worried about having a user "log in" (when login: false) as the anonymous admin just because I tried to stop it being accessible in the web UI in case someone managed to access it when login: true, so my preference was to try and make hidden accounts as inaccessible and unused as possible.

The reason for ensuring there's always an admin account wasn't for this, but just to prevent users from locking themselves out.  Although there's the reset permissions rake task, it was more me trying to be user-friendly.