I guess what I'm trying to say is that, despite the availability and
popularity of the various licenses published by Creative Commons, a Free
Culture activist that's familiar with the issues might still elect to use a
license such as the GFDL or the GPL for their non-documentation and
non-software works for the same reason that a Free Software activist might
elect to use those licenses for their software and documentation: because
those licenses have requirements that the recipient have the *practical*
ability to modify the work.
The licenses published by Creative Commons, while no doubt popular and do say
that the recipient must be able to somehow "modify" the work, don't go on to
say *how* modifiability of the work is practically enforced. That can be a
problem with some things, like a sound file that's made up of many different
tracks and all you get is a single Ogg Vorbis file with everything combined
together under CC BY-SA but none of the stuff that went into making that
file.
There was a long discussion about having such a requirement in version 4 of
the various Creative Commons licenses but, sadly, it's not happening.
