Social Highlighting

I just went into a book I have on the Kindle app for my iPad, and I noticed a new feature: social highlighting. Apparently, you have the ability to turn this on and off (it was on by default), but it shows you what others have highlighted. I wonder how useful this would be for a popular textbook. I imagine someone would highlight just about everything in the book. For this free and obscure book, the highlights were rare and probably useful.