Another nasty variation is that they come with some religion that has a series of logical arguments that can pretty much convince anyone who doesn't have a PhD in rhetoric. So they come along drop off their book of faith and leave.
How is that a bad result? If they arrive, drop off a set of supremely-convincing beliefs, and leave, the only result is that everyone now has the same religion. Everyone having the same religion means no more religious conflicts. As long as the religion isn't "sacrifice your lives for your alien overlords", I'd say it'd be a positive outcome.