Apologies for the fisking.
One could argue that we have little more of an idea whether contributors have agreed to the former, than the latter.
Clicking the save button is enough, legally, to make it GFDL-licensed. This is a non-starter. Furthermore, we are advertised and well-known (as well known as Wikibooks can be :P ) as being GFDL-licensed.
Firstly, there is the inconsistency that the edit page clause doesn't mention the possibility of dual licensing.
Nor should it - it is not the purpose of that system message to educate the user about licensing issues - it's purpose is solely to say "Clicking save means it goes under the GFDL" and nothing more. That's all that's required, and it does it's job.
Secondly, the PD clause isn't on the edit page itself, and that on the book sub-pages, the license is linked to, rather than quoted in full.
This is a major issue, as it's not in edit view. Furthermore, we cannot /require/ contributors to license their work under anything other than the GFDL, as repeated below.
That is, of course, only a valid point as long as Wikibooks is required to obtain users' consent of the license in a particular way.
We are. GFDL. You can multi-license to your heart's content, but all derivatives of your work are GFDL-only on Wikibooks unless the other user(s) agree to other licensing conditions. And we can't force them to do so.
If such a requirement does exist, and states that the clause must be placed on the edit page itself, then I think we can quite easily reconcile this situation by modifying the current clause to mention the possibility of other licensing.
I would be opposed to doing so for reasons explained above. That is not the purpose of that system message. Furthermore, for practical reasons, we can never ensure that any page remains available under anything except the GFDL. In fact, most modules of uim and Scratch are no longer available under their "other" license. Finally, we cannot require users to license their contributions to a book under anything except the GFDL. (You can't say "Your contribs to this book are PD/CC-by-sa - if you don't like it don't edit" for reasons that need no explanation)
I'm surprised that you would think it necessary to delete content that was possibly posted also under a more free a license than GFDL. At worst, the contributions default to GFDL.
The /licensing/ goes on the chopping block - not the book :) Of course it remains GFDL.
Mike