I find feedback mechanisms that allow readers to react to individual parts of text to be useful, especially when the text is of a technical nature.
What I dislike about this particular implementation is the UI: You don't know what happens when you click the button until you have actually clicked it (the same problem has for example the 'kudos' widget on Svbtle[0]). Also the button is slightly obtrusive. Something more subtle with an explanation on hover or a confirmation dialog would IMO do a better job.
A good example of something similar is (the now obsolete) Django Book[1] where people can comment on any paragraph.
[0] e.g. http://500hats.com/late-bloomer/