As someone who uses and is grateful for free and sponsored templates, I try to make sure that the authors' links are intact and work properly. So while I can't tell you how much business those authors ultimately get from those links, I can say that my analytics reports show that visitors to my sites click on those links.
As a user, I do visit those authors' sites to check out their blogs, ask questions, look at what they have to offer, and sometimes buy templates.
I think the key thing is to make sure your theme works really well. If the design is good and the theme doesn't create problems for users, over time you will get plenty of backlinks.