One strength is its good predictive validity, predicts aspects of the babies later development. Research shows that babies and toddlers assessed as type b (secure), have better outcomes both in later childhood and in adulthood, like better achievement in school and less involvement in bullying. They also tend to have better mental health in adulthood. Suggests that it measure something meaningful in a babies development.
It clearly measures something important, however, Kagan suggested that genetically influenced anxiety levels could cause variations in attachment behaviour.