I think it probably has less to do with producing more baby dubs than it does with training a pattern of obedience and a "put the cult first" mentality. That and if you know your spouse has been trained to be loyal to the cult before you, you're more likely to subdue your doubts with an appeal to consequences - you won't let yourself doubt because it'll mean losing your spouse.
Born-ins have proven to be a retention nightmare, and the ones that stay are usually half-assed about it.