Frequently don't have much to listen to while riding except bus and truck engines, but have seen 3, yes three NASTY accidents that the cyclists could have avoided if NOT wearing earphones....first one was the ear phone falling out of the ear and getting caught in the wheel and you can imagine that at 40k an hour....second one, truck coming from behind and rider to far out to the left didn't hear it and didn't get to the side line...and three, rider going down hill and didn't hear other rider shout they were passing...(the third only had ONE ear phone in, but was so involved in the music that wasn't paying attention.
I really think that listening to the sounds of the world help keep me alert and concentrated on the riding...when I swim or run, fine, zone out.