 This is probably fine in a number of cases, folks, but we always prefer having a hard copy of our boarding pass with us. A few years ago while traveling, I had my phone in my pocket and accidentally walked into a large piece of art made of stone at a restaurant, and it destroyed my phone's screen, making it inoperable. Had I not had a hard copy of my itinerary so my boarding pass could be printed at the airport, I would not have been able to board the plane. It's that simple.