The LAST_CALL_ET column in V$SESSION represents, for an ACTIVE, session "represents the elapsed time (in seconds) since the session has become active."
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 :