I think of the disk design more by raid channels matching purpose and separating loads across them. An oltp system I built for a large indie record label a few years ago had 7 channels:

Raid 1 - operating system and apps, main swapfile

Raid 10 - Sql data

Raid 10 - Sql Index

Raid 1 - Logging

Raid 5 - Backups to disk

- plus 2 for the application data / reports etc both Raid 10

Also if specific tables in the database get a hammering they can be partitioned out onto other raid channels.