"What possible reason would ex-mormons have to lie about what they've learned in church, as opposed by what would the church have to lose by allowing the criticisms to go unchallenged?"

You'd be surprised.

1. The most basic motivation that former members would have to lie is simple: revenge. They feel that they were somehow wronged by the church, and their desire for vengeance has outstripped their sense of ethics.

Examples:

*Decker, as mentioned above, was excommunicated from the church for serial adultery.

*Loftes Tryk was tried and convicted in criminal court for a sex crime, and upon receiving his conviction he was excommunicated.

*Grant Palmer was removed from his position as a paid seminary teacher for heterodoxy and professional misconduct (holding his students academically responsible for his own personal opinions) and transferred to a series of less desirable postings within the Church Education System.

2. Another basic motivation is personal insecurity. Although they've justified to themselves why they left, they need to bring others with them as evidence of their being right. For these people, the psychological security inherent in having others come with them justifies lying. This, quite simply, describes quite a few individual critics I've encountered.

3. A third basic reason is financial gain. Within North America, there's a cottage industry for material that is hostile towards the church. All a person has to do is find a few old arguments, put a unique spin on them, find a publisher, and spend the rest of their life doing speaking tours and public appearances. It's rare for a critic of the church to be called out by anyone other than someone who is Mormon, and so few people within mainstream Christianity rarely catch on to how much bull they're being fed.