I believe you are overthinking it a bit, but yes they will overlap a bit. The first thought was to remove the ability to import users the way we currently do (using an action to either pull certain members from a different model or be able to upload a file). There are many customers who are currently doing it this way and ripping the bandaid and removing this feature would have had a negative effect on them. So, we will now have two ways of adding users, the old way as well as via CIM. And if you do it the old way, there will be an API that updates CIM ensuring CIM is current.
Remember, the User Admin role only allows the user the ability to provision/remove a user from a workspace while a WSA has the ability to define the role of that user. In the future, not all WSA's will have the ability to provision users.