At our office we have a file server which is just running Windows 7 hosting out file shares. There primary hard drives are in their own RAID 1 array to help minimize downtime in the case that one hard drive would fail, and all the actual data is stored on a separate RAID 1 array of 1TB hard drives. Everything is backed up onto an external hard drive, which is rotated off-site.
I've seen a lot of people building a basic system using RAID 1 and just use that for all their storage and backup. After all, with a mirror RAID, everything that's on one drive is on the second drive. However, I've seen RAID 1 arrays fail quite frequently (just today again actually) where it corrupts the operating system running on the drives. While you still can get into your data a lot of times and retrieve it when this happens, your system is still down and offline until you reinstall everything again.
If you're going to have a file server I'd still recommend doing a backup to another location off the RAID array, such as to an external hard drive. That way, if the server goes down or your RAID array fails, you just take your external drive to another computer, plug it in, and you have all your data in a matter of minutes.
RAID can definitely be handy, but anymore I don't think of RAID1 for the specific purpose of doing backups. I see that more of as "side effect" of the RAID because if you download a virus, then your backup drive has that virus too. If Windows gets a corruption in the bootloader (which is what I saw just today) then the other drive is the same way. RAID 1 really is just to try and minimize downtime in the case that one hard drive fails and, yes, will make a backup of your data so it's not totally gone in the event a drive dies, but I wouldn't 100% trust it as your only means of backup anymore.
