HI my first post, so please be gentle,
i have been using my 404 page as a,,, "while your here, why not explore the web and my fantastic website," and have a google search box close by... plus ads and of course a way to get back to the main site and a bit of nice content, and links to my other sites, it dosent have to be a blank page you can and should jazz it up to match your sites feel. or even a totally new look.
no one wants error pages of course, but if a link falls out, perhaps it is still in old search results or from link building gone wrong, or if you change your tags about, or even a simple spelling mistake, they do pop up from time to time. your making great use of a dead end, and letting the person know your serious about them and want them to stay on your site or at least give you and your site a second chance,
it also looks a lot better than,
"you go away. 404 your not allowed here"
because they will, and more than likely will never return,
i also tracked my error "page view time" and saw it increased a staggering 1200%, so its more than worth it, CTR was pretty good too,
have a great day
Phillip