Why? To keep users from making use of an improved search tool that gives useful, relevant Web pages an advantage over useless or irrelevant Web pages for a given search string?
As a user, I think this is a great idea. Right now, if Joe User searches Google for something like "pc won't recognize usb widget," he'll get a hodgepodge of results that range from articles on the topic to casual mentions of "pc," usb," and "widget" in a 100-message forum thread. There's no way for Joe to identify the helpful search results without visiting each of the listed pages, and the author of the helpful article on "What to Do When Your PC Won't Recognize Your USB Widget" is competing with a long list of less relevant or useful results. The scenario that Brett describes (mouse over the search results to see what's on the underlying pages) is a win-win-win situation for Google, for the site owners with the most relevant or useful results, and--above all--for Joe the previously frustrated user.