Retina screens in particular are usable at various resolutions. I find myself often switching resolutions on my Retina MacBook Pro, depending on how much screen real estate I need. Which was what inspired me to build the app.
That is true. However, a Retina display has such a high pixel density that many of the scaled resolutions are very usable. In fact, OS X does not even give you an option of using the maximum (native) resolution (2880x1800 on a 15" RMBP). EasyRes does, of course.