Up until December 2009, my main computer was a desktop we'd bought in 2000. At some point, we had upgraded it from 128MB of RAM to 384MB.

That last year or so, things got rough. I was really pushing the limit of what it could run without feeling unreasonably sluggish (with pretty normal workloads of browsing, IM, and either word processing or Ruby coding). First, I switched from Firefox to Opera. Then, I switched from Gnome to Xfce to IceWM. This made a world of difference, and I really didn't miss Gnome. It got me another 6 months+ out of that computer without being terribly frustrated all the time.

In most cases, on a modern computer where you have multiple gigabytes of RAM, I agree with you. But it's still nice to have those tiny window managers around.