These are real concerns, and there were some great responses provided. There seemed to be one answer missing, however. When I joined Bookcrossing lo these many years ago, I did so because I had read about this organization that wanted to increase literacy in the world through the simple medium of leaving a book where someone might find it and read it.
As a great lover of books and someone with an intense desire to have discussions about them with informed and interested people, I found this approach an engagingly simple solution to provide people who might not otherwise have access to free books a chance to explore a new idea.
In addition, when my friends ask why I left that book behind on a bench, it allows me to share this simple but charming idea with them. And, on occasion, inspires them to pick up a book and read.
So, whether you only read on a screen, have too many books already, or just enjoy scattering books, keep Bookcrossing. Who knows? Your simple act may prove to be the inspiration for the next great idea - even if you never know it.