Everybody has different priorities, so I might have stated the obvious. In my mind, people often focus on RAID for the wrong reasons.
RAID is not a replacement for proper backups, and thus is only really a way to improve the availability of a particular dataset. So only doing RAID on the media drive won't make much sense IMHO. I used to have RAID on my media too, and not on the system drive. Then I realized, if the system drive goes bad, I wont have access to my media anyway... (at least not without doing some hardware changes.) Should a media drive go bad, I'll just restore it from backup. It's even easy to restore the backup to another machine if I want to.