I would ask how it ruins it for the next reader. I suppose it depends on the definition of "scribble", but notes in the margins and underlined words are helpful for me when I'm scribbling as it helps me regain the context. Practically, when I borrow/lend a book, I find it interesting on their thoughts, as opposed to bothersome.

It’s distracting and messy.

I’m not reading a book for half-baked insights scribbled in the margins by past readers unknown.