I think Rackspace invests a lot of money into their DNS infrastructure. Yea it's free but its mostly because you're paying so much extra for the servers and support.

No DNS doesn't always mean no one reaches your expensive infrastructure. That may be true for websites relying on lots of random traffic, but most if not all of our customers have been to our site before so there's a strong chance that the DNS has already been cached on their computer or router.