In my experience, storing data in holders that contact the disk is only bad if you pull the disk out often. In that case, absolutely, keep the disks in jewel boxes. If you don't expect to use them that much, however, sleeves in a case are fine. I still have a backup disk of some old OS versions that are way obsolete (hmm, guess I should purge

) but still work. If they are sitting on the shelf and not moving a lot they should not incur much wear. Again, that's my experience, YMMV. Of course make sure that there are not any other disks in the holder that you
do pull out, that movement does cause some wear on the disks that you don't use a lot.
A more interesting problem is data loss through oxidation of the disk, which I
don't have any experience with. Its given rise to gold CDs, gold being resistant to oxidation. As you might expect, its pricey.
Finally, for you ipodders (what are they called?) out there, there is this:
http://www.gizmodo.com/gadgets/portableaudio/podlock-025358.php
I can imaging keeping a 650-700 MB partition on my Ipod (if I had one) and backing up to CD occassionally. Could be a neat solution.