I ride to work every day. Most of my commute is on a popular bike path. I always warn people before I pass them. Every few months, someone decides to do a U-turn or a left turn, without looking, right into my path, while I'm passing them, leading to a near-collision. So far, 100% of the adults who've done this have been wearing headphones. (I've had one kid without headphones do it; I said "on your left" and she got confused and veered left into me. Now I just say "passing.")

Based on this, I think joggers and cyclists with headphones are significantly more dangerous to themselves and others than those without.

Of course, it's possible that you are different and can hear fine through your headphones, but I'm skeptical. (Just as I'm skeptical of people who claim they can drive fine while talking on the phone, or while drunk.)