A website could just put a big "Speed up this application" button that installs the Chrome Frame plugin, and as far as the end-user is concerned, it's just like installing Flash - and they're still "using" internet explorer.
The target audience for this is not the people who are interested in Chrome; it's the people who aren't interested in moving away from IE.
Also, although it is hard to believe, some people actually think IE is a good browser. I don't understand how or why, but some people actually do.
We recently gave up on GWT at work for precisely the reason that IE6 and friends are waaaay too slow to do anything meaningful in GWT. Had we had this option we might have stuck with GWT instead of rewriting in Flex.
Changing browsers requires aquiring new habbits. Most people don't like change. On the other hand, they're already familiar with the concept of a browser plugin thanks to Flash and Accrobat. The Google Chrome Frame can be understood as a speed booster and a bunch of additional features for IE rather than a new browser.
Edit: Icey burn me to it.