I use GeoJSON with LeafletJS to overlay geometry on top of images (using a non-globe project). I wasn't happy with it either, but it works well enough, people write plugins to manipulate it in useful ways, so it's not all bad. It could be useful to add an offset and a scale when exporting to GeoJSON, so people can add it to existing maps.
Viewing post in 0.7.3: Bug fixes, GeoJSON export comments
If someone wanted to generate multiple cities to place them all on a larger map, they would want to adjust the coordinates to match their larger map. The larger map is going to have it's own coordinate system. Normally i think it would use Latitude and Longitude. The map I have uses pixels x,y of the most zoomed out image.
If was going to do it, I would write a script to import the city JSON and adjust things, someone else may just want to concatinate all the cities in one JSON file and use it like that.