I use a separate music player just because I like non-singular points of failure.

The idea of carrying a single object that is your means of communication / music player / camera / train ticket / monetary instrument / entertainment center is neat, until you think about your battery dying / losing it / dropping it and having it crack in half when you're a hundred miles from home.

I guess it is less bad if you tend to travel in packs, and would always have a backup in that way.