I usually have a bookmark so I don't do it in my books but I used to do it alot in magazines. I think it adds to the "lived in" look of physical media and gives a charm.
my students think it's "destroying the book." I think it's the whole point of owning it. @Victoria but they do way worse when they put them in and out of their bags.
i dont mind doing it but people look at me like im crazy!