[JeffF.]

I would think that the Kingdoms in Exile would have kept the stones in nearly continuous usage. Given their knowledge of astronomy (Unfinished Tales Description of Numenor) they must have known about the difference in sunrise and sunset at given longitudes and would have used the stones to see events in far off lands. During normal routine there was probably an agreed upon time where the Stewards of two stones would be scheduled to communicate with each other while the Steward of the Chief Stone in Osgiliath would listen in/observe on. Aside from military uses I would think that their equivalent of scholars, scientists and explorers would have applied for time on the stones to use them for applications in their areas of expertise.

I wonder what special properties the Stone of Amon Sul had? It is described as the chief stone of the North and is known to be physically large but it apparently does not have the same ability as the Stone of Osgiliath yet I find it hard to believe it was merely a large version of the minor stones with no unique properties.