I run with my phone to snap the occasional photo. I used to spend 10min after most runs discussing all the cool things I saw or thought during the run with the wife. Now I can show her an image or two as well. Now that I finally have a smart phone the apps are better than the camera tool: I use strava to log my mileage, calculate my pacing and net climbing, and give me an idea who else runs on my loops and trails.
Sometimes my phone has low batteries, or it looks like rain, or I don't have shorts with a pocket or something. Then I just leave the phone on my dresser and head out the door without it.