I take the subway to work every day at rush hour, and can attest to the dangers of doing this. I see people who are so absorbed in their phone/e-reader/book that they don't take their eyes off them for a minute. Consider a random idiot, staring down at their object of choice.
-standing on the subway, object held in front of them, taking up extra space. Almost falling over every time the train brakes for a station because they aren't holding on to the poles, but just their precious object.
-train gets to their station. Without looking up, they turn and followed the crowd out the door.
-they walk along the platform, onto the escalator, and then stand there, staring at their phone.
-in the ticket hall, they meander through the throngs of people, up the stairs and onto the street.
-off they go, down the sidewalk, still staring at their object, weaving from side to side so those of us who actually walk quickly and look where we're going can't get past.
Seriously, though, they're going to walk off the sidewalk and get hit by a streetcar.