Its not normally the everyday stuff that causes problems, its the guy who dropped and damages his electronic device that thinks its ok,
How many iphones do you see with cracked screens, water damage and people complaining that they need to get it fixed as it dose funny things!
but now is generating interference in all sorts of ways that it was not designed for. The off for take off and landing goes some way to minimise the risk, that is very small of problems. Most common problems would be interference for the crew on the comm radios, and on the Nav Kit. Not what you want on a bad day doing a Cat3 autoland in fog. Risk minimal. But easy to eliminate. Turn it off when asked!
As for ipods and earphones. Off and headphones out for takeoff and landing allows you to hear what is going on and any emergency instructions.