3 disk would be the minimum for a RAID 5 array.
And naturally the reason for using RAID 5 is for the stripe drive speed of RAID 0, with the redundancy of RAID 1, which is why I'm sure is why he was using such a set up. They're supposed to offer security from individual drive failure.
Of course any single RAID solution is no guarantee for the security of all data being stored. If what you have on your array is that irreplaceable or important, your redundant data should also be stored on a remote disk or local server.
Personally, I've always used manual back ups with archive/back up software, but I'll be honest and say those back ups are not generated or refreshed with the type of frequency they should be. Typically just vital files, of which I have multiple copies on separate drives.
The only RAID I use is the SSD RAID 0 boot drive I set up for my graphics workstation. If that ever craps out, I have the original disk image with all my production apps installed along with a clean install of Win7. All other files are backed up on a separate internal HDD as well as removable HDDs for remote work.
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