I've been thinking about it, and I would use a cell phone as an MP3 player. I listen to music on my runs with my MP3 player, but I don't take a phone with me. I don't want to carry both a phone and an MP3 player. I like my MP3 player strapped to my arm so I don't have to carry it and I don't have to have it in my pocket. It is more comfortable that way.
So why would I use a cell phone?
They make arm bands for cell phones now.
I would have a phone with me that would let me call for help in case of a problem.
Cell phones are still getting smaller and lighter.
If my cell phone takes care of it, then there's one less gadget in my life.
Running can be dangerous. If you run long enough sooner or later you will fall. I had a nasty spill when I was in college running a 2 mile loop through the woods. One night after a storm I was running and there was a branch down that I couldn't see in the dark. I tripped at about the farthest out point. When I got up I felt around and saw that there was a huge branch on the ground that I hadn't seen. My leg hurt a lot and when I got back to the dorm my sock was soaked in blood. I had a pretty bad gash on my leg. I didn't go to the doctor for stitches even though I probably should have.
If I had a cell phone, I could have called for some help. It sure would have been nice to walk back knowing I wasn't going to trip over something else. Whenever my cell phone plan is ready to give me a discount on a new cell phone, I'll definitely look into one that can play MP3s.