Pros:
Worldwide unity
Modest sense of hope
Only a limited amount of actual deception (they knew Sin would always come back, but then the people knew that too. Same goes for the death of successful summoners)

Nice topic, by the way. These days it's far too common for angry kids to lash out at all religion, dismissing it as manipulative, evil deception that does not - and cannot - do anything good for anyone. Usually these arguments rely on gross ignorance, or extremely one-sided understanding of history, unfortunately.