IMHO, fate-sharing as a strategy for increasing availability is somewhat underrated.
from rfc 2182 3.3. A Myth Exploded An argument is occasionally made that there is no need for the domain name servers for a domain to be accessible if the hosts in the domain are unreachable. This argument is fallacious. + Clients react differently to inability to resolve than inability to connect, and reactions to the former are not always as desirable. + If the zone is resolvable yet the particular name is not, then a client can discard the transaction rather than retrying and creating undesirable load on the network. + While positive DNS results are usually cached, the lack of a result is not cached. Thus, unnecessary inability to resolve creates an undesirable load on the net. + All names in the zone may not resolve to addresses within the detached network. This becomes more likely over time. Thus a basic assumption of the myth often becomes untrue. It is important that there be nameservers able to be queried, available always, for all forward zones. randy