I haven't flown in situations where many passengers had these, but the ptential downside that I see is what happens when the scan won't take, or when a passenger's phone dies 5 minutes before boarding, ESPECIALLY if the passenger is the only person in a group carrying a phone.. (How does that work, anyway, if you have multiple BP's on one phone? Are they all on the same screen so you can scroll, or do you have to go back and forward through screens to access more than one?)
I'm a belt & suspenders kind of girl when it comes to travel documents. If I was asked to use this I would print out a hardcopy from the kiosk just in case. As I prefer to put my phone away in my carryon when I fly, so that I won't misplace it, I'd rather not have to take it it out for boarding when my hands are full as it is.