Boy, if they run a domain registrar their DNS servers should have better uptime than what I could achieve.

This isn't necessarily true--why?

Domain registrars make money from domain name registration, plain and simple. Registering a domain name basically consists of adding an entry to some text file somewhere, hosted on a server that most registrars have absolutely nothing to do with. Registrars do not necessarily have any experience running systems like DNS.

DNS offered by registrars is a value-added service, one that makes them no money. It's offered because everyone else offers it, and it helps sell domain names.

OK, that's a lie. Registrars hosting DNS can make money, through what is known as "domain parking." Domain parking lets you buy a domain without having a website or hosting for it; you can buy a domain and the registrar keeps it for you, for free! It's very nice of them isn't it? Until you notice that "parked" domains have pages full of advertising, making money for the registrar. The tricky thing here is that with most registars, the nameservers for parked domains and those that answer customer-supplied DNS records are different. Registrars can spend more money on the parked domains' nameservers, those which essentially make them money, than other DNS servers, which don't.

Because registrars can and do often host the DNS for millions of domain names, that means their systems are that much more loaded and susceptible to DoS attacks. Because registrars don't make any money hosting your DNS, they just have to keep their DNS service (barely) working, it doesn't have to be good.