A better method, in my opinion, would be for the RethinkDB devs to contribute a Homebrew formula. That way everything would be installed into /usr/local/ by default, and updates would be a quick "brew upgrade rethinkdb" away. Am I the only one who would prefer that?
UPDATE: It seems they had originally planned on a Homebrew formula but thought the .pkg format was better. Clearly, I disagree with that. Want to offer both? Awesome. But if you're going to pick one or the other, I think Homebrew is the vastly superior method. More here: https://github.com/rethinkdb/rethinkdb/issues/5