So, say you have a site in London and a site in New York. If the link between the two is down and there is only...
a DNS server in NY, despite the fact that the London DC is up and running, only those systems with cached DNS information will be able to find the server. Thus, your AD in London becomes disabled. Of course, if London is a single-broadcast domain, the workstation may still be able to locate the server, but the performance will be greatly degraded. So, if you put a DNS on the DC in London, even though the link to NY goes down, the London DNS is able to serve as locator for all of services the client systems might need.
Now the reason we use DCs is because you can utilize Active Directory zones, which are automatically replicated along with the AD schema information to all DCs.
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