So, if you are running an DML operation, it represents how long the DML has been running.
What if you are running a SELECT that is sending a large number of rows to a client ? If the query runs for 10minutes and you repeatedly query it, does LAST_CALL_ET show the time since the query began ?
What if you are running a PL/SQL Procedure or an Anonymous PL/SQL block which calls one or more SQL statements ? At any instant in time the session may be running one of the SQLs. Does LAST_CALL_ET queried at that instant show the time that SQL has been running ?
Think about it.
UPDATE 01-Feb-14 : Here are two discussions around LAST_CALL_ET :
Three earlier posts by me :