There's an obvious problem with converting them. It would create multiple copies of the same scan. There would be the scanner's released version and the DCM version. Once multiple versions of the same scan are created, it makes it even more difficult to locate upgrades (like paper instead of fiche or rescans). It would change the cryptographic hash function (md5,sha1,tth,etc.) of the file and make it nearly impossible and extremely time consuming to check the integrity against the original.
I'm already in a gray area by upping scans to the site without asking for the scanner's permission (some I can't as they are undetermined or deceased) and even though it would be pretty simple to convert them, it would feel like I was taking too many liberties with someone else's efforts.
That being said, there is a place for converting. I plan on upping a Bumblebee-man scan and he likes (or liked) to put his tags out front, so when I repack it to move the tag page to the end, I'll be glad to use cbz. I have a few more cbzs I can go ahead and add next as well while I contemplate this dilemma.