The principal argument in favor of the GFDL seems
to be "this is the only way we can get our message out".
The GNU Project's motive for using invariant sections is not the issue
here; that's a GNU Project decision, not a Debian decision.
The question at hand is whether Debian should accept or reject
GFDL-covered manuals. The argument for that is that there are many
such manuals and they would be useful to include, and the DFSG can
be interpreted to accept it.