Modern DNS servers (especially resolvers) should have query logging
Since OpenBSD has shifted to using Unbound as their resolving DNS server, we've been in the process of making this shift ourselves as we upgrade, for example, our local OpenBSD-based resolver machines. One of the things this caused me to look into again is what Unbound offers for logging, and this has made me just a little bit grumpy.
So here is my opinion:
Given the modern Internet environment, every DNS server should be capable of doing compact query logging.
By compact query logging, I mean something that logs a single line with the client IP, the DNS lookup, and the resolution result for each query. This logging is especially important for resolving nameservers, because they're the place where you're most likely to want this data.
(How the logging should be done is an interesting question. Sending it to syslog is probably the easiest way; the best is probably to provide a logging plugin interface.)
What you want this for is pretty straightforward: you want to be able to spot and find compromised machines that are trying to talk to their command & control nodes. These machines leave traces in their DNS traffic, so you can use logs of that traffic to try to pick them out (either at the time or later, as you go back through log records). Sometimes what you want to know and search is the hosts and domains being looked up; other times, you want to search and know what IP addresses are coming back (attackers may use fast-flux host names but point them all at the same IPs).
(Quality query logging will allow you relatively fine grained control over what sort of queries from who get logged. For example, you might decide that you're only interested in successful A record lookups and then only for outside domains, not your own ones.)
Query logging for authoritative servers is probably less useful, but I think that it should still be included. You might not turn it on for your public DNS servers, but there are other cases such as internal DNS.
As for Unbound, it can sort of do query logging but it's rather verbose. Although I haven't looked in detail, it seems to be just the sort of thing you'd want when you have to debug DNS name resolution problems, but not at all what you want to deal with if you're trying to do this sort of DNS query monitoring.