My 2c's

To give the consultant his due (???) - maybe he meant the additional RBS that oracle suggests we create in the SYSTEM tablespace, that needs to be taken offline ???

Now, to the point - the SYSTEM RBS can't be taken offline period. However, its usage is controlled internally by oracle. At its (Oracle's) discretion, oracle may use the SYSTEM RBS for normal user operations too (if a situation arises say, when the load on the other RBS's become too much in Oracle's opinion).

You can use it as a normal RBS, the limiting factor here is that the object you are trying to manipulate must be in the SYSTEM tablespace, e.g

CREATE TABLE test (c1 NUMBER) TABLESPACE system;
SET TRANSACTION USE ROLLBACK SEGMENT system;
INSERT INTO test VALUES (1);
COMMIT;

The above will go through. However, the following will not:

CREATE TABLE test (c1 NUMBER) TABLESPACE users;
SET TRANSACTION USE ROLLBACK SEGMENT system;
INSERT INTO test VALUES (1);

---- You get ORA 01552 Error here

-amar