Comment 6 for bug 734781

(In reply to comment #2)
> Is ordering necessary to work around bad servers?

There are some nameservers that drop AAAA requests on the floor. (Not many, and probably a lot fewer than there were a few years ago.) Using such a nameserver when you also have a global IPv6 address (presumably via a tunnel) will cause DNS resolution to become really slow, because you have to wait for glibc to time out on the AAAA request every time.

Presumably you're not likely to find such a broken nameserver listening on an IPv6 address. (Though, this being the internet, there's probably at least one, somewhere out there.) So that would be a point in favor of putting the IPv6 servers first. Not sure how it weighs against the "more likely to have broken IPv6 than IPv4" point.