[bittermang]

"Custom 404 pages of often quite confusing as they will try to be clever and redirect you to other content that may be interesting."

I agree, however I also believe that is the intent of filing it under "usability". It isn't usability as you would commonly define it, a good UX, but rather keeping the UX of the site consistent across all states, even failure, and giving the user an entry point back in to the rest of the site. A default Apache 404 does not do this, it's just a flat white page, with your only option being to go back from whence you came. If that wasn't your site, then the perception is you've lost a potential visitor, and that potentially could've been avoided with a custom 404 page.