I do the same (write on a piece of scrap paper). Perversely, I like getting books with notes in them, though. I guess I'm just a Nosy Nora and like peeking into other people's lives.
I was always taught to "respect" books - no dog-ears, no notes, leave it pristine and as new-looking as possible. Since much of my reading material came from the library, this was even more important.
I tend not to write in my books either, preferring to make notes in a notebook or on sticky notes if they are available. Sometimes I get desperate though and then I use pencil and keep my markings to a minimum.