Having all your data backed up in real time and not having to ever copy from one drive to another or wonder where the #$%#% a file went: priceless.
Raid5 is not backup, it's fault tolerance. Raid failures occur (trust me, I've blown 2 r5's, one from simulataneous drive failure).
My current rig is 8x160's (gotta break down and up them to the 400's like you have), 1 is a hot spare. That is copied (not real time) to a raid5 6x80 array (gonna move the 160's there after I upgrade the main array).
Then, the original is also copied off box to a Lacie 1tb drive on a linksys nas controller.
I also replicate remotely to another Lacie 1tb drive at my cabin.
Thats backup (I hope I hope I hope....) 😉