1) It's not tied to any other services (so I can switch, say, webhosts without having to move my DNS config)
2) It lets me configure timeouts so I can experiment with settings without waiting forever.
For these I shell out the $30/year for DynDNS.com. Totally worth it to not have to worry about this sort of thing. I also get dynamic DNS if I want to configure a home subdomain, which is built into most Linksys routers.
I also have a DynDNS account for my Linksys/DD-WRT router. And I agree on having DNS configuration decoupled from my other servies - years ago I used to run my own bind server.
I don't want to sound cheap, because $30/y is totally affordable, but providers give you DNS for free (zoneedit, dnsexit, freedns, ..) - I'm trying to figure out which one is liked best on HN. But I'll note the DynDNS option, because you are right in saying that 30 bucks for a good service is next to nothing.
And thanks for the advice on freedns.
I used to use it years ago for a home server. When I was doing that I think I preferred it over the other ones I had previously used (dyndns comes to mind).