Why we have a moral responsibility not to criticize praying

Because there's not much fundamental difference between prayer and meditation; the biggest is that in the former you think someone's listening. You don't have to do anything differently for meditation than you do for prayer. I'm not sure which of the "surprising" benefits you think would be unique to prayer. 

I'd say that accounts for a vanishingly small minority of believers. Further, there can be negatives when you substitute a belief in God for doing practical things that could make a bigger impact on whatever it is you want to accomplish; ie, praying for healing VS getting medical treatment.