Don't know. Not entirely sure what the ggplot method was accomplishing. For simplicity, I'm trying to understand the functions that case study uses. For instance, it manipulates the graphic itself with that image package it uses. Frankly, this is probably because it is trying to transfer over what the RgoogleMaps plotting function does to the ggplot plotting structure. That requires some tweaking. The difference, I would suspect, is that the RgoogleMaps is limited in what you can do compared to what using other libraries can offer, but that takes work. For instance, if you wanted to generate the heat maps and topomaps, that the case study does, you're gonna have to be able to plot those lines and colors onto the map, which requires that your use of lines and points and other functions are in the same parameter space as your map canvas. This is something brought up in the RgoogleMaps vignette, if I recall correctly, but it references using another package, also.
For a simple pin map (plotting points-to-locations), there's no reason to use all the bells and whistles when the one package handles it all. But you will notice that RgoogleMaps uses the png library. I assume what it does is very simple: read an image raster in as a grid. It is then doing something similar to what the case study did using its own image processing. Frankly, I'd like to see this done in a more generic fashion that can be more easily translated to base-r. I'd also understand more what is going on!