I was running this morning when I pondered this very question. Would I have been able to make it up the hill if Disturbed wasn't screaming in my ear? Would I have been able to continue without Breaking Benjamin, Cher, or Audioslave? I like listening to music as I run. It does get me moving and keep my going. It also sometimes becomes "white noise" - I don't focus on the sound per se, but it does free up my mind for other thoughts.
What did those folks in the 70's do when running became such a trend? Listen to the birds chirp and the wind blow in the trees? No nature for me - I have a couple of hours worth of my favorite music on a machine the size of a binder clip. I love technology!