When I got married three months ago, all our guests, to my knowledge, had cell phones, and maybe half had phones capable of taking pictures; many others had digital cameras. They did take tons of photos, but they were in no way a distraction from the event.
We did set up a site (weddingturtle.com sends you 500(!) free business cards with your personal URL so people can upload photos of your event; we set them out on the guestbook table) and were very grateful when 5 guests uploaded about 400 photos, many from unexpected angles and exactly one silly shot of someone posing with a camera in front of their face.
My friends and family did not need to be told not to pay attention to their phones instead of the actual events of the day. If I thought a person needed to be told, I might choose to ignore their behavior rather than confront them in advance, but I can see why someone might do that.
FWIW, I found our photographer (pro, but a family member doing it for free) much more of a distraction than anyone's quick snapshots! I wanted to be hanging out with all my guests, and there he was, having us pose for just one more group shot! I'm definitely immensely grateful for all those photos today, but "best day of my life" and "standing still and smiling till my cheeks hurt" aren't necessarily synonymous to me!