At the risk of re-opening an old argument (but, of course, we won't do that
), I see Creative Commons as an entirely a creator thing. Most readers won't care beyond the fact that they can read it for free.
This creator's perspective: I personally have little interest in trying to expand on other people's creative works*. This probably explains why I have never given something like the Creative Commons licence much thought.
But now, having thought about it for a while since you started this thread, I still don't find it an attractive option. Even if someone else wanted to build on my work, I don't think I'd encourage them.
I think it would be different if it was a deliberate collaboration: lots of discussion and give and take, getting a feel for what was being created and having some control over it. But just opening it up to anyone? Not for me. After I'm dead, fine. I won't care. But while I'm alive I want my worlds to remain my territory. I don't really want anyone else messing around in there. It's personal.
* Which is not being critical of those that enjoy such activity. I have read some excellent fan-fiction and some of it is extremely clever. It's just not something that interests me very much. I have done some collaborative work in software development, but I much prefer to work on my own. You can imagine my report card from school: Not a team player; doesn't work well with others. 