It all boils down to this: if it's a personal book that I don't intend to share with anyone ever, then I'll scribble all I want¹.
If I think it could be read by others in the future or it's obviously meant to be read by others (library loan) then don't.
And instead take notes on the side, with page reference if you really want to refer back to a specific page at some point.
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¹ There's always a risk that a nosy kid finds my scribbled advanced-potion-making book in the future, but what's the worst that could happen?
I always assume someone will inherit my books, either when I die, or because I gave them away as gifts or exchanged them for other books at a used books shop.
This is my stance by default, but let's say there's a book I love which I never intend to part with, even then I would feel bad about crossing words out. After all, if I love the book, what's this "violent" disagreement/scribbling about?
How would someone writing in their copy of a book impact your ability to enjoy your own copy? Relax, it's their property and it's not criminal, so let them do what they want instead of trying to dictate rules.
If it's from a book shop, you can thumb through it and choose to not buy it. If it's inherited or a gift, then what's the harm? Just discard it if it's been "ruined" or shelve it if there's some sentimental reason to keep it, and get a clean copy to read.
> It's obvious from context I'm speaking of books that ended up with me,
I doubt any of my books will end up with you nor will the books of the vast majority of HN readers and commenters, so why should any of us care about your peculiarities and avoid doing something that works for us and that we want to do?
Sometimes you can thumb through and discover flaws, sometimes you cannot. Sometimes you can get that exact edition in better conditions, sometimes you cannot. Sometimes you buy a lot of used books, and cannot check every one of them. It's always a disappointment to find a book with pages all scribbled over.
> I doubt any of my books will end up with you
You're being too literal here.
If you don't care about my "peculiarities", just don't engage with me. Someone mentioned their attitude towards scribbling, and I cared enough to offer a counterargument on why I dislike it.