I think that the mechanics make it possible for a religion to grow into something that people care about and respect. From what I have heard about SA, people truly do gain from joining. Minor knights gain access to promotions within their realm. A powerful ruler can gain the aid of a block of powerful nations. SA can declare a crusade against a nation or religion without fear that they will be torn up when the rest of the island decided to tear them up for being a bad religion. This is all possible, however, because some very capable people have politiced their way into having the support of rulers, councils and entire nations. They have a lot of influence over secular dealings.
My opinion, however, is that the odds of another religion managing to accomplish anything like that is slim. The closest that one might come is a state religion. Even then, the fact that the religion and stat coincide does not make it much different than simply being in a realm without much influence. I think that if we want more religions to have the clout to be openly hostile, prejudiced, and all that, we need to gice religions better tools and most are simple unable to come close to those things without some help.
Just my initial thoughts...
