I used to think I was a multi-tasker until a good friend told me that there's no such thing as doing two things at the same time. And then I thought about it: whether you're watching TV while doing laundry, or talking on the phone and making dinner, you're either half-doing two things, or not getting anything done at all.
When I first started riding my bike about three years ago I used to wear one earbud in my right ear. Work, errands, you name it and in it would go. I wouldn't recommend it to anyone, honestly. Actually, about a week into doing so, I simply refused to distract myself so carelessly.
I recently read in a popular magazine that listening to music will greatly amp up your workout output. In a gym, sure. I think most people can see that controlled environments and music can go hand in hand. But when you're going anywhere from 10 to 20 miles through space on a two-wheeled device - to me it's best to have as much control as you can over an environment that constantly changes every hundred feet.
I often wish for a distraction when I first get on my bicycle, whether commuting or training. Thankfully about five minutes in, the scenery changes and I start thinking about things - my daily to-do list, politics, pop culture, self-stuff, etc. - and need for distraction is no longer. It's not worth the crash. The likelihood of crashing is already high enough without, but that's another subject entirely...