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.
Also exporting with coordinates in meters could be useful.
Ah! So you are talking about ability to specify a scale and an offset somewhere in the generator to get absolute coordinates in json, right? Well, if someone can parse GeoJSON (manually, using a tool or a library), I'm sure they can adjust coordinates as it fits their needs :)