Plenty of people have their phone on them when they run, for numerous practical reasons, chiefly safety. e.g. I use Spotify so I need my phone for my music, and I live in central London so I generally run from home base to somewhere to do something (lunch, shopping, meeting friends for a drink etc.) and Tube it home, so I want my phone on me when I get to where I'm going. The Apple Watch will replace my Garmin, I can see it already.
If you're in the gym then you probably want as little clutter on you as possible and might only want some tunes and the accelerometer in the device, to use with the workouts app. A watch plus some bluetooth headphones would be perfect, even better than the Shuffle, which is the best gym device currently.