Essentially the thought behnd not using RAID5 is that the access time of RAID1+0 is faster and still provides redundency. It's same with SATA vs SAS, SAS, iSCSI, Fiber, and such have a much lower latency for file access. This is due to the mail being stored as many millions of tiny little files.
That being said, I have 250 users running on a 4 drive SATAII 3.0gb/s system in RAID 1+0. It works quite adequately, faster with a pile of 6 SAS drives is planned next.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now