The sphere administrator does not run the hardware of the cell. This task is still accomplished by the cell administrator. The cell administrator is not overwhelmed by thousands of user request. An individual principal may belong to many spheres without having to change the DCE identity. The cell administrator acts as backup to the sphere administrators.
Spheres solve the following problems.
A sphere query to the DCEcp now only has to return a much smaller number of users that are in one particular sphere. The list limitation of DCEcp is mitigated. A sphere administrator can only modify user passwords in one particular sphere. You don't have to grant cell privileges to all sphere administrators. The management of the cell is now distributed among many spheres administrators.