I think that good choice depends on required performance, capacity, and data value stored on protected disks. RAID is just one type of protection and other protections should be used. Other data protections are:
- Local Disk Replication (Volume Clones - Business Continous Volumes like i.e. EMC SnapView)
- Remote Disk Replications (EMC SAN Copy, EMC SRDF, IBM Metro Mirror, etc.)
So sometimes RAID 5 or 6 makes sense.