So yes, overuse is not healthy. But consider another angle: for me a smartphone can fill useless time like waiting for an appointment, the bus, etc. instead of wasting their time these people text their loved ones, read articles about self employment or check reddit for cat pictures.
Giving someone not the attention they might deserve is indeed rude. But i hope customs will change hopefully to the better and people put their smartphones down when appropriate.
But i don't think it is right other without knowing the context and what they're doing.
Edit: I think i want a better mute button for my phone and a hardware mute button for my laptop. Everything should be hidden besides calls from circa 5 people and the third call from one number. Besides that the OS should not show me any notification, message,etc. not even the notification light should blink and when i check the time on the phone i should not see how many messages i missed