> relying on something simply thrown up there on the web ... They don't feel like being left to your whims should you choose to change license

Fair enough. Naive question... Does the Wikimedia Commons (and other established sites) not address this? Licensees of CC material will find content and license change histories, archiving, and (thus) audibility if a dispute arises. i.e. claims like "this was the license on date X" become verifiable