I have always had a strong feeling I should keep my iTunes library externally - could you say more why you believe as you do? A product like this might be very useful for that indeed.
1. External drives mean that if your internal drive fails, you still have that data separate.
2. Your internal drive is usually smaller and is more expensive to upgrade. An external can easily be upgraded. And I have always bought an enclosure and a bare drive but deals at places like Costco make either option viable.
3. As you upgrade your machine, you simply copy the iTunes data library file to the user folder on your new machine and point iTunes to the external drive.
4. And while I use Time Machine to back up my internal drive, I use a second external drive along with Carbon Copy Cloner to backup my iTunes drive. So that I only need a backup as large as my media drive and don't weigh down my Time Machine backups. I also use CCC to clone in incremental backups, so that if anything gets accidentally deleted, I don't lose it. Which is why for this I think a mirror is a bad idea. I had originally looked into a Drobo, but they have poor performance and reliability from what I have read, at least they did when I was considering options a few years ago.
It may sound like a lot, a media drive, and media backup and a Time Machine backup, but after years of not backing up, I chose to get realistic about my needs and didn't want to lose all that data, from music and movies to iPhoto libraries with family photos.
Now iTunes match and iCloud do make it very easy to not need a physical backup of your data as the match and Cloud make it so you can redownload anything that goes missing, but not if Apple stops carrying it in their catalog, or in my case, a large part of both my music and movie libraries are from old media and not purchased through iTunes. I also don't currently use iTunes match.