Faith and "Truth vs Utility"

[chimp3]

I have often heard people say that the only good reason to believe something is if it is shown as likely to be true.  What if, however, you have a proposition for which it is impossible to show evidence either for or against it's truth value, but also for which there is great utility in adopting?  Is adopting this kind of proposition as if it's true just as good as (if not even better than) adopting a proposition that is demonstrably true?  If the proposition is "God exists", I think many theists might answer yes to the question, but I'm not sure.

Looking at this from another angle, the utility of your belief may serve someone else more than you. Your belief that god may heal your disease might serve the miracle worker more than you. You receive false hope and the fraudulent healer drives off in a Rolls Royce. 

Your belief in an unfalsifiable premise may harm others. 72 virgins and a ticket to Paradise does no good for the victims of the faith based explosion you caused.