You can already do that with a non-contained database - just give nobody else access to the database.

As I understand it, the main beneift of contained databases is that they have no dependency on annything outside the database, so it's very easy to move a database to a different server. With an uncontained database, that is hell - users will be orphaned, if the new server has a different collation, you'll get all kinds of problems in your code, and lots more. A fully contained database should not have any of those problems.