Swallow's use of that plot device was necessary if he was to portray Valeris as a sympathetic character. Perhaps it would have been more "interesting" if she was a coldly calculating menace, but that also has been done in other novels.
I didn't walk away from Cast No Shadow
with any inkling of Valeris as a sympathetic character, and while I am not the author, I can't see how the idea of making her sympathetic ever entered into his mind. (He is free to correct me if I am wrong.) The conclusion of her story in the book makes the fact that she isn't a sympathetic character crystal clear.