It's hard to understand the exact connection between a document and the work
that can be made with it.
Even if it's under a NC license, The work made by it can be licensed as
completely free software. At least it should be possible.
But since you mention it, there's probably some legal aspect that prevents
such a thing to happen.
Either way, the quote I took about XKCD's license is from the FSF website,
so they clearly don't recommend it as a general rule, as explained in
Calinou's link.
And yeah, I didn't think about practical related issues.
I guess some contextual exception should ideally be stated on that FSF
licenses page,
like "such NC license (maybe only specific ones btw) remain ethical only for
specific types of works (artistic), because *explanation* ".
Not as badly stated of course.
