I like seeing little notes from prior readers. References to other works, question marks, and angry exclamation points are all fun to see.
Underlining or highlighting can be interesting to see what the other reader thought was important. Highlighters can be distracting when they overdo it, but even then it can be fun to see where the highlighting tapers off were the prior reader just gave up.
But dog ears or times when someone folded a whole page in half is just painful.
I don't imagine myself to be the final owner of the things, but I am definitely the current user--and chances are the future user, and writing in the book makes the use better. Somebody may wear some of my clothes someday. Does that mean I can't have them altered for better fit?
Underlining or highlighting can be interesting to see what the other reader thought was important. Highlighters can be distracting when they overdo it, but even then it can be fun to see where the highlighting tapers off were the prior reader just gave up.
But dog ears or times when someone folded a whole page in half is just painful.