Did you not see the 'Make Promise' option? That all but screams that she
is going to corrupt someone eventually, whether she necessarily intends to or not. Savin's
tacitly confirmed this. So some people might decide to not give her the option in the first place and give her the boot. Also, she's not
necessary to any of the wayfort refurbishments provided you accept Ryn's offer or have Demonl'yn, and some people just might not like her to begin with and see no reason to let her stay.
Also, for people who play the game with a roleplaying angle, there are any number of reasons why they might think 'Hmmm, you know what,
no' when faced with Farrah.