I asked this question also during the Nov 26 EMEA Office hours. I understood the overwhelming majority of responses to be the the Departments and the OBS you see list of OBS's are two different things.
Glad to see that you seem to share my understanding if I am not mistaken.
Since then I've seen this happen even when the number of levels is less than then.
Say the number of levels is 6 and then the Departments are reorganized so that there is only four levels..
The OBS still shows 6 and there nothing the user can do. Though this case is "supported"
When it is not a financial structure are the extra levels once created remaining of is there some sort of clean up system which removes them?