A picture tells a thousand words. Lets look at what happens when Google Chrome visits a web page that does not exist:
What Are The Issues?
1. Revenue from 404 pages down. Although that depends on if you park or develop yourself.
2. Look what happens when its http://www.secure-domain-blah.com/user.php?user=john&session=12345678. Google is now in possession of my User ID [...]

