Now you (Alessandro) described how even FDL without invariant sections and cover texts can be harmful (because you can't prevent others from adding some).
None of what Alessandro described is harmful, it is explcitly allowed by the license. You are claiming that there are problems when there are none, it is like having people claiming that the GPL has problems by disallowing a GPLed project being converted into a non-free program.
Now the FSF can, of course, by the FDL, include the new content into their distribution, but not without also adding the new invariant sections. What would they do?
Same thing they would do if they wanted to include GPLed code into Emacs: require a copyright transfer. This is required for GNU projects, and should be required for _any_ and _all_ projects. That people think that there has to be some kind of a `free for all' is absurd.
If you wish to have a way to enforce the copyright, then you will need to have copyright papers. Otherwise, you cannot enforce it at all, it really doesn't matter what license you are using. A judge would love to hear "Sorry your Honour, but I couldn't gather the copyright holders to actually sue the accused'.
This is a misconception you, and others have shown repatedly when it comes to copyleft, that unless you have a single copyright holder (or a very small number), it is impractible to enforce the license. And if you cannot enforce the license, the license looses its charm.
In theory, anyone can go and make Linux a non-free program, since it is simply impossible to enforce the license there.