I distinguish highway riding from street riding. On the highway you don't hear anything anyways, and there's a relatively small number of variables to pay attention to. To keep my mind active during long stretches on the highway, I enjoy listening to music. Also, the right type of headphones can help block out ambient wind noise. If I'm on the highway and not listening to music, then I use earplugs.
Street riding requires more attention (there's more variables to track), is entertaining enough on its own, and you actually can hear things going on around you, so I don't listen to music. I waver on whether or not to use plugs on surface roads.