Are you a spine breaker? Or a dog-earer? Do you expect to keep your books in pristine condition even after you have read them? Does watching other readers bend the cover all the way round make you flinch or squeal in pain?
I do dog ear my books, and I have also been known to use them as coasters. I used to be much rougher on them than I am now- I don't bend the spines back as far or crack them as much- but I have no expectation that my books, paperbacks especially, will remain in pristine condition. I don't go out of my way to trash them, but for the most part I'm more attached to the stories or the ideas in my books than I am to the physical objects themselves. I have certain rare or collectible books that I take good care of though. It doesn't matter to me what other people do to their books, unless they're library books from my library!