Imagine this case :
We have number of campus network.
In Each network .. there is server farm
we want that every one want to access server, will need to be Authenticated.
If we put the AAA at the "center" : when a link between campus is down, the AAA will failed and user can not go anywhere (Internet and internal server)
If we put UM in each network : we need to duplicate userdatabase in all UM, since user have to able to login from anywhere.
So .. I think the needed solution is UM have tobe able to syncro eachother .. or UM have tobe able to use external database as backend.
If Mikrotik will put feature for external database (i.e: Mysql/PgSQL) ... it will be nice .. we can leave the syncro task to the backend database