There are a lot of reasons I encourage people to walk without headphones in their ears. They mostly stem from three:
1. You stay aware of your environment and therefore safer (from cars, animals, and people)
2. You can help people... sometimes they need directions or a seat or... you'll never know if you don't open up your ears
3. You learn all sorts of things!
There are many days when you just want to zone and music seems like a great way to shut out the world. This is okay; I do it too. It's just important to remember what you're doing and make it a conscious decision instead of a default action.
Today I received the ultimate reward for keeping my ears open. As I was walking from the subway to my house, an East Harlem couple was walking in front of me. We were walking in the same direction (they live around the corner, I guess) and all of a sudden the guy starts singing. He had an AMAZING voice and his range... sigh. He was singing something that sounded like a mix of 70s soul and gospel and it was fabulous. The female with him joined in, with background bits, and I could've followed them for miles. They sounded like the angels about which they sung.