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.
This sentiment is problematic. It is basically saying that you can
put restrictions on free software that prevent it from being ported to
proprietary platforms.