They cost too much IMO, both resources and gold. Also, Principes have strategic move 2, Emerald Guards have 1.
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Great indebtedness does not make men grateful, but vengeful; and if a little charity is not forgotten, it turns into a gnawing worm.
I find Communion slave more useful for the *low*-level mages. 4 communion slaves and the 1S1H mages can cast Smite, without any research. This also doesn't put much fatigue stress on the communicants, so they don't die so much. Plus they're not old.
To get decent mileage from the high-level mages I find I need two communicants for each mage and some mid-level research. Quite a bit more gold and a substantial delay.