At least the article could have addressed why that would not work for their use case.
edit: also now they have to classify polygons into groups that wholly fit into some "city" polygon. That seems like it becomes a pain to maintain since you may want to expand a region in the future and it happens to go outside of your previously defined "city". Or maybe you want a region that isn't in a "city", like a "country" or "mid-atlantic" region.