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.