We only need one headphone... but we NEED it as much as breathing air.
Personally I think it's "impolite" to listen to music outside. I mean, if someone wants to say hello to you, you will have to take the time to remove your IEMs just to hear him, and it will make him feel uncomfortable about the "disturbance" he caused. If the person notices you're wearing headphone he will be less likely to interact with you, and vice-versa, when I look at people listening to their music in the bus I feel them more closed-off.
Of course when I'm reading a book in a library or focused on a task, my life-code permits me to listen to headphones in such scenarios.
In an ideal world noise wouldn't exist, so we could all use open-back headphones whenever and wherever we want to... in a non-ideal world my second headphone is a noise-isolating headphone.
So I have one high-dollar, essential-to-my-happiness open-back headphone, and one random 5$ IEM for some infrequent odd scenarios when I'm on a secret mission and I need to hear orders in noisy environments. I don't even like to use IEMs (though I do prefer them to closed headphones), every time I'm forced to do so I'm wishing the world was noise-free and peaceful and less stressed.
Oh and I almost always have 2 pairs of safety earplugs with me or in my schoolbag.
Edited by devouringone3 - 5/8/13 at 11:44pm