You said, "Healing is reserved for rare and special people. Advice is withheld and frequently veiled. You get delivery missions that are next to impossible. It seems once you decide to follow her, you're in for a world of pain."
Sounds like the Catholic religion to me. There is no promise that God is going to help you through disease, poverty, or the trials of life. However, you serve because of a moral code and a belief that there is a greater power and a greater good. At the end, you get to sit in God's presence and the reward is eternal happiness in Heaven.
What is in it for the worshipper? Getting in on the plan, maybe. Being able to follow and look after the ones that you love. Maybe you enter a meld where you are part of a greater consciousness with the power to direct the real-world agents of your Goddess in order to make the world a better place. Either way, the need to find a bigger meaning to an isolated life is branded into the human genome. I'm comfortable with Behira being a standard that people are willing to follow.
That and she's cool. She collects rocks. She likes dragons. She kicks the ass of evil.
I gave my priests a goddess that can be measured, experienced, and who rewards her followers with magic. Not half bad. I don't need 90 pages to justify this.