Unfortunately third-party MAS apps can only add items to the menu bar, first come first served. The last item is added on the left. You have to run EasyRes before the other apps to get its menu bar item to appear first. Apple's system menu bar items appear on the right-hand side, and can be moved around using command-drag. We don't get access to that, unless we use private APIs (like iStat Menus does) but then we cannot distribute via the Mac App Store.
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.
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.