Regardless of your views on privacy you will likely agree that some non-identifying data keeping is ok. Things such as how many times an online journal has been used or how many times a book has been circulated. This data can be crucial in decision making. An article from OCLC in a recent Library Journal discusses this. They include some examples from WorldCat.
The good news is that libraries don't come to this task empty-handed but with rich, structured information about the materials in our collections. Librarians possess many other valuable data sources, like circulation statistics, reference transactions, interlibrary loan (ILL) requests, usage behaviors, and more. In our daily work, libraries create a wealth of information that offers an extremely rich perspective on the characteristics and use of the wide variety of materials in library collections.