Moderation is the key I would agree. I also think texting and driving should be prosecuted harder than drunk driving, but that's a different debate all together.
Pulling up your smartphone mid conversation is rude, nobody with any social awareness is going to argue against that; but maybe
You Might want to self evaluate how often you are in fact the person interjecting themselves into conversation and being the rude one.
If people are always on their smartphones as you say, then odds are they were on it moments before you started talking to them having a conversation with someone else. Isn't a little rude to just burst in and demand a person's undivided attention simply because you're in the room now and want to talk?
I can't count the number of times I have snuck out back for my precious 15 break at work to get a quick cigarette and check on my fantasy football team only to have it completely derailed by some dumb co worker wanting to join me and ramble on about their surgery, kids, or some other subject I quite frankly couldn't give a flying fuck less about. Perhaps I should be more assertive but "uh huh yeah" while staring into my screen is my polite way of saying "shut the hell up and go away, I don't want to talk to you right now".
It's amazing the number people that complain about smartphones destroying communication that can't interprete such little social cues. I've seriously turned it into a game; how many times I can say "uh huh yeah" before someone realizes I don't care what they are talking about (21 times is the all time record in case you were curious). If you seriously desire face to face conversation that badly maybe you could I don't know...ASK THEM A FUCKING QUESTION as opposed to rambling on with your story. There is a simple solution that nobody thinks about.
If every once in a while somebody zones out mid conversation and starts texting that's rude. If it's happening ALL the time, odds are YOU are the problem. People have probably been ignoring you you're entire life, you're just now noticing because they have a physical device to show you they aren't listening.