Gods use level scaling? This must be Oblivion.
I'm kidding, I do like your answer. It seems appropriate. So, why not use the Numidium against Redguardia land?
Maybe there was some kind of risk inherent in using it? It does seem to go out of control and go on city-stomping rampages from time to time, maybe Septim was worried that overuse would cause it to become increasingly unstable, and it would destroy places he wanted left intact? Just guessing at the moment, of course, but...