[bradleyland]

RAID and replication are good for protecting against hardware failure, but hardware failure actually accounts for only a handful of the reasons you want a backup. RAID, replication, and backups are all solutions to problems. Once you look at the problems instead of the solutions, your answer becomes clear.

Often, the need for a backup arises from the need to go "back in time". RAID and replication offer no solution for this. Ok, so depending upon your replication setup, it may offer some help, but the steps required to go back in time usually involve starting from a recent backup, and using files involved in the replication to "replay" recent changes to get you back to the moment in the past you wish to return to.