Just how big Google is as an Internet presence never fails to surprise. It is really, really big.
An example for you - a site previously indexed by Google was removed from their listings due to its owner fighting a losing battle against comment spam (story over at Slashdot).
As a result over 10,000 visitors a day stopped coming to the site (Javalobby) as they couldn't find it when searching. With mass market Internet users browsing via the search engine rather than the old ways of the directly typed URL, the bookmark and the hyperlink from other sites, such a de-listing can be catastrophic for any site.
And with Google counting for anything from 70% to 78% of the search market, it would be fatal not to remain on side with them.