Is it worth it to use these? Or does it cause too many followers to abandon the faith to outweigh the benefits? I would think that if you use it in regions where your religion is in total dominance, then it wouldn't matter how many followers you lose because they would just get converted back anyway. Otherwise other religions could come and take them up.
They're very much worth it. It's a standard strategy for Ibladesh to use the influence options to maintain regions and/or spread hatred and/or prepare for a (religious) take over. The most easy regions to do this is where the region lord is a follower of the faith (in Ibladesh, that are all regions). The amount of followers quickly restores to the highest possible for that region.
I do believe though that a higher Oratory skill will give better results. And yes, sometimes you loose a lot of followers. Sometimes you don't. It's like preaching, really.
