One huge problem with the CREW formula is that not every book which is read is checked out. This is particularly a problem when there is a cluster of good books together, which makes doing research in the library much easier than taking the books home. I spent literally weeks (maybe months) in the QC 790 section of the Dirac library and never once checked out a book, but I can name you the authors of many of those books off the top of my head: Artsimovich, Miyamoto, Trivelpiece, Stacey, etc.

Sure, if you know that the library is planning on getting rid of a book you love, you can check it out to let them know they should keep it. But how is a library user supposed to know which books the library is planning on getting rid of? Should I check out the entire QC 790 section?

Many times I have had the experience of going back to a library to check something in a book I had read before, only to find that book is gone. At that point its too late. You can't even buy the book from them. It's just gone.