I think there's a discussion to be had about whether it's a legitimate goal for a free software license to rule out proprietary formats such as word documents. But I think it's quite clear that the GFDL does rule out using word documents as source -- though the recent GR confuses thissomewhat.he fact that it is or not legitimate
For the fact that it is or not legitimate to restrict free document to open format; I would say that IMHO it is at least acceptable since otherwise it would make it unusable by someone who have decided to use only free softwares. Another consequence would be that a derivative work of a free document suitable for the main section could not be suitable anymore for the main section (a document with a propriatary format depend on a nonfree software). It seems clearly in the siprit of copyleft that you can require that all derivative works which qualify for the main section still qualify for the main section.
Olive