I find the article very interesting as memory usage is a topic that pops up regularly on our mailing lists. I’d just like to point out a small detail, regarding the methodology used for measuring “plain” desktop usage with real life apps.
Both OpenOffice.org and Mozilla Thunderbird used with Xfce to measure its memory usage, are quite heavyweight applications. Usually, people use lighter alternatives with Xfce, like Abiword. Abiword can be plain GTK+ and doesn’t require GNOME libs (it can make use of GNOME libs, but there are not mandatory and Abiword can compile and operate w/out).
Same goes with Thunderbird, people sometime prefer a plain GTK+ mailer like Sylpheed or Sylpheed-Claws which are very capable mailers instead of Thunderbird. With these apps, you can run plain GTK+ apps w/out KDE and GNOME libs.
Still, I agree that people who want to use a Web browser will probably choose Firefox. But similarly a lot of people I know use Thunderbird, OpenOffice.org or Firefox with either KDE or GNOME too.