[JeffF.]

One of the points made in UT was that Denethor was not trained in the use of the stones therefore the strain on him was greater. i think that each warden of a stone would have had assistants to minimize the strain on each individual using them. The special training would probably result in a special, secret (from the public) guild whose loyalty to the kings was proven and who were given many years of training. The stones were too valuable to use for mere communications, and after all they are called 'seeing stones' not talking stones. It would have been irresponsible to not use them. In fact there may have been too much dependence on them, the INTEL chiefs of the kings may have preferred info from what was seen rather than what traders, explorers, sailors, scouts and spies reported (rather like our own dependence on spie satellites in the mid 70's when we allowed other sources of INTEL to be dismantled).