Although it may seem that you should back up online redo logs along with the datafiles and control file, this technique is dangerous. You should not back up online redo logs for the following reasons:
* The best method for protecting the online logs against media failure is by multiplexing them, that is, having multiple log members in each group, on different disks and disk controllers.
* If your database is in ARCHIVELOG mode, then the archiver is already archiving the filled redo logs.
* If your database is in NOARCHIVELOG mode, then the only type of backups that you should perform are closed, consistent, whole database backups. The files in this type of backup are all consistent and do not need recovery, so the online logs are not needed.
* You may accidentally restore backups of online redo logs while not intending to, thereby corrupting the database.
Link: http://docs.oracle.com/cd/A84870_01/doc/server.816/a76993/backupst.htm#425901