In general, I think that we should of course try to have as much packages on ftp.debian.org as possible. We should try to have all packages in ftp.debian.org and try to discourgae the use of any kind of secondary dump site (atleast, this is how it was before). But on the other hand, ftp.gnuab.org is well known in the Debian GNU/Hurd community and supported. The intention of alpha.gnu.org at least was to have it not known, and supported as little as possible.