I disagree. I love buying used books and seeing what other people found interesting, whether it's by way of notes in the margins, highlights, etc. I also love it when people write their name in the front of the book and when they read it. I like the idea of a single copy of a book making its way among many people, all adding their own thoughts.
Same reason I miss those library cards found in the back of books. I enjoyed seeing the history a book had, how years could pass without anyone checking it out or seeing a flurry of checkouts.
If only people did this with care. I have seen too many books where people just highlight everything.