That the WT only contains AS or female channelers who try to become AS. To make a novice, who failed in this, a WT servant seems completely redundant. It makes more sense to send her away, and then let this woman decide on her own what she wants to do with her life. As often said, it's anyway absurd to think that such a woman would be a crucial resource in the fight against the Shadow. And if the premise were just to save the life or ability to channel of any channeler till TG, one would also have to forbid the Accepted/AS tests, since some burn out or even die during these tests (or for that matter any other dangerous tasks). But actually those tests are important to shape a female channeler into a capable women, who actually could help against the Shadow. However, it's laughable to blame Siuan for the decision to send Else away, if it's simply a result of the education system, namely that a person who doesn't want to learn is fired. Any Amyrlin would do this.