Makes me think of a potential sploit for it. Cast it and then demand outrageous tribute from everyone that isn't pleased but is also not your worst enemy. They decide whether or not to grant such tribute based on your relative power ratings, right?
Anyone ever tried this?
How is "power" calculated anyway? Does each unit have a weight of its own? Is base str the only weight factor or do promotions count as well? How do spellcasting units factor into this? Summons should not affect the power rating themselves but an army of mages capable of summoning 100 fireballs at once should be recognized as pretty powerful.

Power is a seperate attribute on each unit so we can move them around if we decide that archmages should contribute more to a civs power graph and such. But it isn't dynamic (it is added once when the unit is created, and subtracted from when the unit dies) so as implemented it couldnt be effected by level, promotions, etc.