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The DNS lookup only has to be done once and is then cached on the users local machine, at least for the overall request (if not longer).

Thus the cost is only 160 bytes for the page, which isn't all that much.

Additionally assuming their cache timeouts are non-trivial there are several caches to reduce the delay incurred by this.

That's another thing YSlow and PageSpeed have complained about for ages, but hopefully the local resolver cache mitigates it somewhat.

Wouldn't that be cached after the first load?

Yes it would be.

Doesn't that depend on the browser? Chromium definitely does cache DNS records (chrome://net-internals/#dns). I remember Firefox relying on a system configured DNS cache (dnsmasq or pdnsd on Linux).