I will be ordering my Pi3 in the next couple of days so will create a vanilla image using RetroPie 3.6 and OpenELEC 15.2 / 16. I’ll post a link soon as it’s ready.
As for “what is the advantage of a dual boot setup vs standard single partition setup” – well when I first made my dual boot setup it was the only way to guarantee a stable KODI experience by using OpenELEC rather than the Raspbian installer version of KODI (which had bugs at that time). You cannot install OpenELEC alongside RetroPie (as far as I know), so it has to be in its own partition. It makes for a rock solid KODI experience with a dedicated RetroPie partition with only it’s own filesystem layout and I can have a nice menu to be able to switch between the two OS’s seamlessly. It may not be for everyone, but for me it’s perfect. Its also the only way you can use Plex and RetroPie together (as far as I know). Not that I use Plex, but a lot of folk were looking for a solution to that.
Hope that answers your question! I guess If you’re having no issues with the Rasbian installer version of KODI then there’s no need for a dual boot build. I prefer OpenELEC so this is my only option. It was also a good Linux learning tool for me along the way!
All the best