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.
The arguments appear to be:
1) There are many GFDL manuals.
2) The many GFDL manuals would be useful to include.
That's two parts out of the three I mentioned, and the third part is
crucial. The DFSG doesn't directly say anything against the
requirements of the GFDL. I sent another longer message explaining
why.