Because the boot drive would usually be the drive with the most random transactional use on a workstation. Depending on the card / DAS / SAN, the transactional & performance overheads of RAID5 vs RAID0 for an easily backed up boot drive / array isn't in most cases an effective tradeoff for a high-performance workstation.
If it's a server, sure. But if it's a workstation - unless constant uptime is more important than performance - you only really need to keep data on a RAID5 DAS / SAN, and backup the boot drive/array properly.
Since the Mac Pro isn't quite as flexible as a real workstation (no SAS support out of the box, etc) and falls more into overpowered desktop territory that's what I bought/use it as - Now I just have 10K drives in RAID0. However on my Windows workstations I have 15K SAS drives in RAID0 and the PC's themselves connected to a RAID5 SAN, which is the optimum configuration for performance + safeguard of data. A similar but lower-performance combo is possible using XServe RAID in RAID5 / Pro + 10K SATA striping, which would offer best-as-possible Pro performance along with data protection.
On a tangent but I'm pretty surprised by pricing for Xserve RAIDs as I was initially by the Pro, very tempting indeed especially as they're purported to be cross-platform.
