I built a kick-ass machine for a friend yesterday, and my co-worker berated me for not using RAID (and specifically RAID 10). I didn't do it with my DAW primarily because of the cost (but also power consumption/heat/noise and I think it is SERIOUS overkill for R10), and because I backup the main data drive and project drive anyway. And I didn't feel at the time, and still don't now, that there/is was an appreciable performance gain.
 
With 3 separate drives, the risk is mitigated somewhat, though I can see the potential value. But besides time, losing the Operating system drive isn't catastrophic because nothing is on it. I can see a RAID1 potentially replacing the project and sample drives, but still not the OS drive.
 
Let's discuss!