Hard disks as backup media
Solution 1:
Disks=faster, cheaper, larger capacity
Optical=data should last longer
Solution 2:
Some data (documents for example) must be stored in an immutable form. You can't beat some sort of WORM or optical system can't be beat. If that requirement doesn't exist for you (check with your legal department and any records managers you may have) I think you can't beat a hard disk.