Other than that, I agree that books want to be handled. I read a lot of books and I also re-read a lot, and my books look worn. I also accept bending pages to remember where you left the story; I never have bookmarks handy.
But scribbling? No, you're banned from my house.
It's recognized as a study aid. Especially if you're re-reading or studying a lot of material on a subject and need to go back. You can easily see what your past self was thinking or maybe quickly find a part you found interesting.
It's fundamentally useful to write up your own books.
It is just one tool among many for the acquisition and retention of knowledge.
I like used books with notes in it. It's interesting to see what someone previously reading felt was important or to glimpse into the past life of book itself.
I’m not reading a book for half-baked insights scribbled in the margins by past readers unknown.
Plus "your own books" are never truly just your own. In the example from the author of TFA, they are his family's. I wouldn't have wanted my brother's or parent's scribblings in books from our family bookshelves!
I decided about 20 years ago I wasn't going to use bookmarks anymore. If I can't find where I was, I need to reread from whatever I recognize. If I can't figure out a place to drop in and understand what's going on, I should just start over from the beginning (it'll probably be faster the second time anyway.) I also really hate dogears; they crack off in about 10 years or so.