It's not really about the smartphones, I think. They are just a symptom of much more fundamental behavior (which may or may not be problematic - depending on how you look at it).

All those 5-10 second breaks -- waiting for a light, waiting for an elevator... If it were not for the phone, what would you typically do? Focus on some random thought or two. Then snap out of it and carry on. The fact that now this is often not a random thought but random input from random app doesn't seem to change that much - pattern stayed the same, just now you can see it in other people from the outside, so it seems more profound.