Some people still use 640x480 because they have no idea how to change their resolution and it's the default resolution. And for example, my school uses some odd resolution somewhere between 640x480 and 800x600, because the teacher running it has no idea what he's doing.
Now really your right about very few people using 640x480, but what about higher resolutions that are getting more popular like 1200x1600, 1280x960, or 1280x1024? The text might look small and when they try to make the text bigger it screws up the layout. My monitor is a 21" which is getting a lot more popular and people set it on larger resolutions (I use 1024x768 BECAUSE you people who design website for 800x600 and 1024x768 ONLY and I use the web a lot. I could set it higher but it screws up the web pages.) And the LCD monitors only come in one resolution so for those LCD's with 1280x1024 they can't even change it!