The following shows the first line of the output of DESCRIBE ST_GEOMETRY from SQL*Plus:
SQL> DESCRIBE ST_GEOMETRY
ST_GEOMETRY is NOT FINAL
'NOT FINAL' is a PL/SQL keyword that refers to whether or not an Oracle datatype can serve as a superclass which can have subclasses. By specifying NOT FINAL, it is possible to define instantiatable subclasses of ST_Geomety such as ST_Point, ST_Polygon, etc.
If NOT FINAL was not specified, Oracle defaults to FINAL which would prohibit subclasses.
NOT FINAL does not have anything to do with the completeness of ST_Geometry itself. The ST_Geometry datatype and functions that ship with ArcGIS 9.2 are complete and fully functional.