Here's your answer.Please someone tell me there was some damn point to the saint and she reappears later in the plot?
The only thing that would have made me react and ask them to please clarify, was the when part, not "if you die" but "when you die".Well i agree, having a "we will resurrect you when you die" option make it very very appealing.
( you could also die and have them resurrect you X years later allowing you to live in the futur )
More frustratingly, she WAS intended to become a member. The WN did a fairly okay job at it, but the LN (I assume) and subsequently the manga botched that idea.
Everything up until Dorothy shows up followed the WN. Here, Dorothy was to have a deep conversation with Rael, learning her background, reason she freaks out, why she was so against the demon lord. Yes it was mostly talk, but in the end it was her diplomatic skill that won the saintess over, and Rael went on to learn more about Tissé, Iris down the line.