All things being equal (money, space, etc), would you rather own copies of the books you read? Or borrow them?
The "etcetera" in the question would be having the time to catalog and shelve my books — or the money to hire someone — to keep them orderly so I'd be able to lay my hands on exactly the book I need at exactly the moment I need to refer to it. As it is, even keeping track of my notes from some of the excellent books is a problem. Thus, my desk piles up with stacks of paper and book notes.