Depends how you're serving them. They can have value from various factors. For instance how the person views the external page. Does he allow 3rd party cookies? Does he have a toolbar that monitors his preferences? Is there a mechanism that reports these access to the images so it counts towards you in the search results?
Then how does your server sends the images? Does it use thumbnails or a lower res image so you conserve b/w? Is it a straight image access or it passes through a thumbnailer etc.
And if you decide to block them how do you block them? 403? 301? There is a difference. You can make them count in favor of your site.
Finally how do you detect the hot-linking? Is it referrer based or some other method? I have seen sites bringing up a broken layout because I block the referrer when I browse online. I only see one page then I am out of course.