There is a reasonable chance that the print is available on the very device it is needed to unlock (the phone screen, for example), perhaps enough for a thief who snatches your phone to have a decent chance.
Assuming they have a laser printer, photoshop skills/subscription and know about acetate sheets. I'm willing to bet most phone snatchers don't even have one of those ingredients.
Most phone snatchers aren't after the information on the phone. The point is not that this would not be a targeted attack (it would be), but that it might not require a particular combination of circumstances or set-up to follow one around and snap just-the-right-angle photo.