The word count for the world as a whole, and individual articles is great. It seems like a word count for each folder to see where the overall wordcount is easily separated would be a useful feature, especially to determine where work is needed, and to see how much is done in certain areas.
This feature seems that is could be useful, as it is already incorporated in other aspects of the world-anvil interface, and could be a helpful tool, that hopefully would be a relatively easy implementation.
As far as other uses, it would be beneficial for worlds with multiple people working on them, who have goals per person to have an idea of how the workload for the world is being divided.