Let's face it. The ride in can be boring and an mp3 player or radio can really break up the time. On the flip side you won't here that truck running the red light.
There is a lot to be said for not wearing headphones. Just like wearing a helmet, using a light at night, and signaling your turns, being able to hear is a feel-good way to ride. I rely heavily on my ears to warn me about cars behind me or approaching hard-to-see corners. Also, if you are in a confrontation with a motorist, they might get you for wearing the headphones, just as they were on their cell phone.
Alright, there are some times and places where headphones might be OK. Riding on your stand during the winter, obviously is no problem with music. Some people rock the 'phones on public bike trails. The argument goes that because there are no cars, there is less we need to listen for. I would say, sure, maybe, but when I go to pass you, I will call out, "on your left" and if you can't hear me and swerve and hit me, I am going to be really angry. My worry with listening to something on a straight, boring bicycle path, is that people zone out. This can obviously be dangerous. I have heard more than one horror story about people wearing headphones on bike trails. I know I have passed people, calling out hello before I pass, and the people still look surprised to see me.
Some folks wear headphones but at a very low level. If I wear headphones, I keep them way turned down. If I cannot hear my bike, they are too loud. This can be ok, depending on your riding environment. Not wearing headphones is best, still, as you can still zone out even at a low volume. Another problem is that if you are in an accident with a car, they can still get on you about wearing them.
Just one Headphone!
Here is a solution that might make both sides happy. Plus you get to look like a secret service agent. One side will obviously be blocked but it is infinitely better than both ears plugged up. Even better, get a phone/mp3 player/radio and use your hands free for all of them. The cons for this derivation are a much lighter shade of headphones quietly. 'Nough said.
It sounds absurd, but I have definitely heard a few people riding, especially on the trails, with headphones so loud I could hear what they were listening to. Obviously all of the previous cons apply to this: zoning out, not hearing contra-traffic, problems with drivers after an accident, etc. I really don't see any positive to listening to your music that loud on a bike. Go for a walk if you need to have it that loud.
Tips about headphones and other ways to be cool on your bicycle.
VA has a law against riding with two headphones in, I will try to find out about more states.
Sad news stories about accidents with headphone-wearers.