Id never do it on a borrowed/library book, but for personal books why not? You arent exactly doing significant damage to it and it's rather unlikely that any given random book is going to significantly hold it's value or be worthwhile on a historical timeframe or anything like that. Books are made to be read, and I kind of like how well loved books get dog eared and spine-creased over time.

I see it as a sign of a well loved book. Having said that, I wouldn't do it to a library book or someone else's book.