[Sombre]

I don't think the ability makes them useless at all. As long as you don't take big squads of them trekking around in your own dominion all they will do is nix your dom in one or two provinces where you keep them. They go well with raiding armies, sieging armies etc and also work very well with negative scales, since they reduce the amount you'll suffer from them. As long as you keep a squad of priests preaching in your cap you don't really have to worry about domkill either - plus your heretics will nix enemy dom coming towards you.