Whenever I'd contemplated systems like this I had assumed that the communities and the user account stuff would be seperate to one another. It seems off to have first order and second order communities based on the instance you join. I think it would make more sense to join an instance that has no real communities (other than stuff like instance related news) and then connect to the instances that have communities you care about. I'm a bit too tired to articulate why exactly right now. I would not be surprised if we end up there eventually though even if it's not enforced by design.