recovery question

After the online backup, lets say there is no transaction therefore no log switchs nor checkpoints occurs.so checkpoint scn of the datafile and controlfiles should be same and database must be consistent.still need recovery?

there are always transactions always if the database is up and running, you have transactions.

ops$tkyte%ORA10GR2> @test
ops$tkyte%ORA10GR2> select dbms_flashback.get_system_change_number from dual;

GET_SYSTEM_CHANGE_NUMBER
------------------------
65882054

ops$tkyte%ORA10GR2> exec dbms_lock.sleep(3)

PL/SQL procedure successfully completed.

ops$tkyte%ORA10GR2> select dbms_flashback.get_system_change_number from dual;

GET_SYSTEM_CHANGE_NUMBER
------------------------
65882055

the database is ALWAYS doing something. and yes, you would need recovery there are things happening before, during and after your hot backups - always.