[ido]

So by the same measure do you also support legislation to disallow rural Americans from moving to the cities? How about those from sparser cities into denser ones?

    So by what metric does increasing population 
    help those of us who live here

Not everything is about helping you. Why do people in HN complain about zoning/density restrictions in SFBA? Keeping real estate restricted/expensive is beneficial to people who already own a house there!

   areas that are about as packed as most of 
   Western Europe: LA, SF, Boston through DC,
   and about a dozen other massive metro areas.

http://www.newgeography.com/content/002808-world-urban-areas... - only 2 of the top 20 metros are in the US and they have the lowest density (table 1). Also from that page: "the least dense urban areas with more than 2.5 million population are all in the United States."

The US is sparse in any meaningful metric.