There have been several debates online recently in regards to whether we should be listening to music while running (or cycling) or not. One of the more recent debates is in this article from Runner's World. So I thought I'd weigh in with my own thoughts.
The arguments for either side are fairly straightforward.
The arguments against listening to music include the fact that headphones can block other noises, such as cars, trains, etc., thereby increasing risks to the runner because they are unable to react to warning sounds, such as a honking horn or oncoming vehicle. While headphones are no more insulating than being inside a car listening to music (and cars these days are very well insulated from the sounds of the outside world), a runner or cyclist is obviously much more vulnerable than someone in a car.
Another argument against is that we, as humans, typically set our pace according to the beat of the music we're listening to. Therefore, your performance is somewhat determined by the pace of the music. This can either be a good thing (if it's pushing your pace a little bit faster) or a bad thing (if it's slowing you down). Either way, the purists will tell you that you should let your body set its own pace.
The arguments for listening to music are more personal. Some people are simply not engaged without music. I admit, sometimes I need music or something upbeat to keep me focused. Listening to music also provides a way to escape. If you run or ride for a release or escape, then music might be exactly what you need.
So ultimately, it comes down to what motivates you and keeps you going.
For me, I am mostly indifferent. Sometimes I love my music during my runs. I often run on trails where my risk of being run over by a vehicle are incredibly low. I often choose upbeat playlists for my runs so I think it helps me push my pace a little bit, or at least keeps me from slowing towards the latter part of my runs. Without music, sometimes my mind wanders and I don't get to clear my head in the same way as I do with music.
Other times however, I love leaving my headphones at home and simply listening to the sound of my feet hitting the gravel, the birds chirping around me, and the wind rustling the leaves. When I run at night, I like to hear what's around me so the time of day definitely factors into my decisions.
Mostly, I go with my gut in the moments before I head out on a run.