RAID1 essentially IS a backup. All your data is stored on two seperate drives, any data written to one, is written instantly to the second. This is better than just backing it up to a second drive every few weeks, wouldn't you say?
RAID0 is the exact opposite of a backup. It actually puts your data at a higher risk of loss.
RAID5/6 is not a substitute for a real backup. However, it is better than no backup at all.
Rig1: System Specs.
Rig2: A8-5600K@4.4GHz / AsRock FM2A75 Pro4 / 8GB Corsair DDR3-1600 9-9-9-24 / HD7560D / Samsung DVD-Burner / 1.5TB WD Green + 3x3TB WD RED in RAID5
Rig3: Athlon X2 4200+ / M4A79 Deluxe / 4GB G.Skill Pi DDR2-800 4-4-4-12 / GT430 / Sony DVD-Burner / 500GB WD
Rig4: Phenom II x6 1605T @ 3.6GHz / Asus M5A99X Evo / 8GB PNY DDR3-1600 9-9-9 / GTX470 & GTX470 / Samsung DVD-Burner / 1.5TB Seagate