Well, this does not sound like a good idea to me. You are not protected from disk failure while the mirror rebuilds. There is a chance that your master disk may develop a bad block just then, even if the RAID controller does a periodical surface check on RAID 1 disks -- and some controllers can't do that at all.
The safe way would be to add the third disk to the RAID1 set, let it rebuild, then remove it again.
Back to the original question, advantages of separate RAID1 and RAID5 sets:
you can have different types of disks for OS and data. In a SATA setup this could be WD Raptors for the OS and 500G+ disks for the data. O.K., I probably wouldn't do that with an exchange data store.
You can put the data volume under control of a volume manager. Logical volume management for boot volumes tends to complicate things a lot.
With the proper volume management software, you can migrate the data space later while keeping the OS disks in place.