That's one heck of an edge case! How does fsck manage to completely confuse filesystem metadata and data like this? Surely this is only if there's corruption in the (outer) filesystem, right?
I've not looked at reiserfs in many years though, so I could be mis-remembering here.
Yeah... especially if you tend to back up systems when you're rebuilding or replacing them by just backing up the whole disk. It's got everything, you can mount it as a loop device (which you can't do with a compressed image as someone else mentions), etc. I've got a lot of random filesystem images around, and once I trashed a fs entirely with Reiser images on Reiser, I was done with it.