Over at Cre8asite Forums there is a fun thread named I'm Feeling Lucky. The thread has a poll asking, "What do you guess is the proportion of Google searches done with the [I'm feeling lucky] button?" Which all the respondents reply to as below 5%. You and I know, that no one ever uses it. So why is it there?
Is the "I am Feeling Lucky" button used? Not really but they don't know what it does. They asked the user, should we take it away? The user said no way! Its fun and they keep it there for that reasons.
The Cre8asite thread quotes from a blog that basically says the same thing (Mayer from Google was quoted in my coverage at SES);
4. The infamous "I feel lucky" is nearly never used. However, in trials it was found that removing it would somehow reduce the Google experience. Users wanted it kept. It was a comfort button.