What functionality is missing? What is unsatisfying with the current situation?

Currently for UserStates we can only choose if someone isn't logged in, is a follower, or isn't a follower. Being that I use Subscriber Groups and userstates as a huge part of what I am designing, it would be insanely useful to have a way to hide/show content if they aren't a subscriber or are a subscriber.  

How does this feature request address the current situation?

This would help with making the presentation of worlds much simpler. Want to show people a specific welcome when they aren't a subscriber for a better call to action? You can with this. Want to hide all of that when they are a subscriber and show people the rest of the world once they have followed the call to action? Now you can.  

What are other uses for this feature request?

This would make creating streamlined Calls to action much simpler and make presentation much cleaner. Currently you could just assume that anyone who isn't a follower is not a subscriber either, but what if they follow and don't subscribe? Or what if they subscribe but don't follow. the call to action is now hidden because they meet the criteria, but they technically didn't subscribe like you want them to.