I tested in 11g without RMAN catalog. When CONTROLFILE AUTOBACKUP is OFF and you issue backup database; without any additional option, it adds current controlfile and spfile into current backup set. But, when you set
RMAN> CONFIGURE CONTROLFILE AUTOBACKUP ON;
then issue just
RMAN> BACKUP DATABASE;
it'll back up data files without Control File and SPFILE. Upon finishing the main backup RMAN runs again backup for Control File and SPFILE itself and backs up them into AUTOBACKUP folder. It's important because after main backup completes it adds information about the last backup into control file and backs it up with latest changes. Without this feature Control File inside backup set will not include your latest backup information.
If you set CONTROLFILE AUTOBACKUP ON and want to include an old Control File during backup, force it with option INCLUDE CURRENT CONTROLFILE:
RMAN> BACKUP DATABASE INCLUDE CURRENT CONTROLFILE;