Rybczynski is correct in using the phrase "proponents of downtown living" and illustrates one of the issues.
It seem to me that such a term "proponent" suggests something which should you should do because it's good for you, like eating spinach. Why be a "proponent" and just enjoy living there? But Rybczynski has it right.
An awful lot of the public discussion about cities is driven by fears -- "sustainability" and "peak oil." Downtown = dense. Dense = using less oil. Using less oil = dealing with peak oil. And so on.
As someone who simply likes and enjoys cities, such downtown living proponents strike me as weak allies. The best reason to like cities is because of pleasure, not fear.