Why shouldn't I create a new user on Oracle SYSTEM Tabespace?
The Oracle Documentation states:


  The default setting for the default tablespaces of all users is the
  SYSTEM tablespace. If a user does not create objects, and has no
  privileges to do so, then this default setting is fine. However, if a
  user is likely to create any type of object, then you should
  specifically assign the user a default tablespace, such as the USERS
  tablespace. Using a tablespace other than SYSTEM reduces contention
  between data dictionary objects and user objects for the same data
  files. In general, do not store user data in the SYSTEM tablespace.


The documentation also explains how to create an alternative tablespace etc.