Jimmy Wales wrote:
(There can be edge cases, of course, but they are a bit difficult to construct.)
They're not that difficult to construct, though. One example that Gregory pointed out is an illustrated textbook or how-to manual. A loose interpretation whereby a copyleft image can be used in a non-copyleft work would allow a textbook publisher to lift all the illustrations from this free-content textbook for their own proprietary textbook without having to release anything back.
Of course in that case, the tie-in between the text and the illustrations might be sufficiently tight that a court would rule it a derivative work anyway, regardless of what the Creative Commons organization says about the matter. In my view it makes a big difference how closely integrated the text and images are---a newspaper article about John Doe that just happens to have an image of John Doe accompanying it is quite different from an illustrated instruction manual.
-Mark