I personally love the quiet when I'm out in the mountains and don't listen to music. I am also easily distracted so I don't believe I could listen to headphones on the road (it would be bad for me and others around me).

But my hubby rides everywhere and he's using one earbud and leaves the other out. He is the most alert rider I know even with one ear with music. It seems to depend on how alert you are as an individual.

Unfortunatley even those that know they are not alert still wear headphones.