Simo:
If in a document I say: "I think Free Software is a good thing!" and I licence the document under GPL, you are entitle to change (without even notify me) it to be: "I think Free Software is a BAD thing!".
Now do you think this is fair? What is the meaning of permitting other people to change MY toughts?
Mark:
Quoting: a) You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.
Rather, it is clearly marked as a combination of your thoughts and theirs. In that, even the above edit is accurately described.
True. Still, there is a great difference. A program usually has little "philosophy" in it, it's mostly practical work. If a derived program doesn't work properly people just doesn't use it and will most likely be able to retrieve the original program by the original author. There is little damage to the original author if the modified program doesn't work.
On the other hand, one can read a modified document and not understand that it "doesn't work". Such a document can be printed on paper, so the reader can't readily get to the original document. And even if there is "prominent notice" of every file that has been changed, it's rarely read.
So I agree with Simo that the GPL isn't really suited to more-than-trivial documentation works, and invariant sections are an important feature. This even though I agree with Mark that the "prominent notice" part of the GPL is a very important issue, even though it's too often forgotten.
I also think that "verbatim copying is permitted" is the best license for non-documentation writings.
[PS: I know perfectly that my own use of the FDL hasn't been completely consistent with these ideas. I know my errors: raising them here won't lead to anything. If anyone wants more details please write me offlist]
/alessandro