Because such restrictions make sense, you don't need the right to modify my thoughts about why I wrote the book, or to whom I dedicated the book.
As this seems to be a center of your argument, can you back this up (to use your favourite expression)?
Do you really need the right to modify my essay about how I simply love chocholate cookies, thoughts which I want to share with the world?
I'd think that the right _not_ to modify such a piece is self evident
Why should a dedication of a manual deserve different restrictions than one of a computer program?
Because a dedication isn't a computer program? A computer program thrives on being modified. A dedication doesn't.
You might say because a computer program is functional. But so is the main part of a manual (as has been pointed out to you).
And as I have also pointed out. This is exactly why only secondary sections can be invariants.
So the question can be reformulated as: Why must a computer program be 100% functional, and a manual doesn't have to be?
Becasue the English language allows for writting non-functional pieces. Can you explain why you need the right to modify my poem about dragons? And why I shouldn't be allowed to attach it to a manual?
Technical differences don't really apply in most cases. An invariant part could be included in most kinds of programs without hindering their normal performance, such as being shown by a particular option or menu item, or even on a startup screen (that a user who doesn't want to read it can click through, just like a manual reader(*) can skip over the pages with the invariant sections).
Alas, a menu item or a startup screen changes the _behaviour_ of the program, and you should always be allowed to change the behaviour of the program. A manual doesn't have a `behaviour'.
Secondly, the question whether someone should be allowed to modify your expression (or your "thoughts", as you prefer to call it), is beside the point. Even a GPL work can be accompanied by an unmodifiable text (GPL, paragraph 2, last sentence).
I think you are misreading that paragraph. It is about aggregation, and not putting specific bits into a GPLed work. I.e. you can put GPLed software and non-free software on the same CD, but you cannot combine those two into one work.
There are two main differences compared to an FDL work with invariant sections:
1. The former case would be two different licenses (GPL plus something else), the latter case would be "all FDL". But that's really just naming, as the FDL already gives two rather different sets of rights for the main part and secondary sections.
2. In the FDL case, the invariant sections are tied to the main part.
They aren't tied to the main part, infact, they cannot be tied to the main part in anyway or form.
You can't reuse parts of the latter without including the former. In the GPL+x case, you can. That's IMHO the main difference, so I'd like to know why you think having to include them is a good thing.
Having an "invariant section" in a program, makes it impossible to modify it so that it does something you want (say that it has a GUI, and you wish to remove this GUI and make it into a library, if the GUI splash screen was an `invariant section', then you would never be allowed to do this). I think we can agree on this.
Having an invariant section in a manual, doesn't cause the same dilema. You can still modify the technical content of the manual so that it is synced with the program. And still be able to make an manual that you can use.
Cheers.