This is a graphical representation of the cognitive trap.
To understand the trap, let's look at the log-on states of a single site.
As you can see, the Y state doesn't exist. However, when another site is added we can look at the multiple site log-on states.
Here, the Y state does exist, because it makes sense to be able to be logged into the network without having an explicit user on the 2nd site.
This is the correct representation without the trap.
As you can see, the C user role doesn't have to automatically create a user account on the 2nd site.
The feature request is to create a Visitor profile. Somebody that can visit any site on the StackExchange network without being logged out. The benefits of having this profile are 1) No 'Welcome to Q&A' banner and 2) One click account creation.
1) is self-evident. 2) maybe it's a bug, but when I click on 'log in' on a 2nd site, I never have my account created. I always have to type my OpenID+Password.