As I see it, she refuses to let them sacrifice another teenager for their goals, one who knows nothing and is essentially just another victim. She's already a victim, and she's well aware of what she's getting into. Plus, she apparently has some power on her side, not to mention likely her own people who are loyal to her rather than the priesthood. Yes, her being a glamoured teenager is a little bit Deus Ex Machina, but I still liked it.