My thoughts on the whole headphones thing:
As usual, it depends. In traffic situations, you need all your attention focused on the situation. Traffic, BTW, can include everything from Boston streets to a crowded bike path. I wouldn't feel safe listening to tunes there.
In less congested situations, it might be OK, especially if you are using the music to set a tempo for your spin, although a metronome on your bars could work just as well.
In any case, leaving the traffic side ear uncovered is probably a good idea.
I'd use headphones in certain situations. DH and I have been discussing having 2-way radios on in town so we don't have to scream at each other. (Car back! Pothole! Glass!) These would also be handy on a tour, since we don't always ride at the same pace. (Stopping for a picture. Car back.) I could argue that they are a safety device, as we are both watching out for each other.
I've also thought that on tour through long stretches, say the sandhills of Nebraska, a recorded book would enhance the experience. Preferably a book that has something to do with the area you are touring. That could be just nice.
Give big space to the festive dog that make sport in the roadway. Avoid entanglement with your wheel spoke.
(Sign in Japan)
1978 Raleigh Gran Prix
2003 EZ Sport AX