i feel google's advantage of the "I'm feeling lucky" button is two fold.
1st - it contributes towards google branding. when google first launched people thought it was a cool idea and thus added to google's "cool" factor. since it only appears on the homepage and google's hompage is very clean it is not really hurting anything. besides advanced searchers tend to use search toolbars more and more which makes the "im feeling lucky" button less significant.
2nd - by keeping that button there it starts discussions about the button and deflects internet marketers attention from how to reverse engineer the google algo and serps.
instead of asking why does google still have the "im feeling lucky" button it might be more profitable to discuss how google instant results has made the lucky button irrelevant since as soon as you start entering a search term the button disappears as an instant serp is displayed. maybe it would be more profitable to discuss why when i turn off instant search the lucky button sends me to a different site from what i see ranking number #1 when use the regular search button?