CWM CAN NOT FLASH KERNEL IN RECOVERY.
4ext takes care of it. You don't HAVE to buy the app. 4ext also has wipe menu with format all partitions (except sd card) option which works the same as superwipe.
Rule of thumb, its always best to backup and wipe whenever you switch between ROMs for a clean install (unless u are flashing an update of your existing ROM). It doesn't take more than 6 minutes to backup and wipe. It's always worth it and you can always advance restore /data from a backup if needed
Sent from my iced out HTC Ruby