Have a look at /etc/init.d/halt. This file surely belongs in /etc/rc0.d - and the only thing it defines is the stop functionality (i.e. /etc/init.d/halt start is a no-op). We can hence assume that scripts in /etc/rc0.d are called with the stop rather than start argument.
Then having a look at /etc/init.d/networking, we see:
stop) check_network_file_systems check_network_swap log_action_begin_msg "Deconfiguring network interfaces" if ifdown -a --exclude=lo $verbose; then log_action_end_msg $? else log_action_end_msg $? fi ;; which first makes sure that no filesystems are mounted on the network anymore (which should be the case after umountnfs.sh ran) and then disables networking (via ifdown) to, for example, release DHCP leases or anything else sensibly one would want to do upon network deconfiguration.
To conclude, having a link to /etc/init.d/networking in /etc/rc0.d is perfectly fine.
(Note that I use Debian, but the scripts should essentially be the same.)
Furthermore, you should normally use update-rc.d, which is able of enabling/disabling services, either for specific runlevels or in general, and ls in order to see which services are enabled/disabled. I guess Ubuntu won’t switch to systemd any time soon, although one can apparently install it in Debian already, if one choses to do so.