What do you do if another disk in the array fails before the rebuild onto the on-line spare is complete? Start scrabbling for backups, that's what!

The justification for RAID 6 is something that is becoming much significant as the capacity of disks goes on doubling whilst the access speed doesn't change. I leave you to think on how long it's going to take to rebuild a 1000GB hot standby in a RAID 5. You are TOTALLY vulnerable for that time.