I keep my phone in the pocket of my riding pants. If the bike and I get separated in an accident, I want the phone accessible. Also, I don't have a large data plan, plus some of the remote areas in the western US don't have cell phone service. And, I like building rides in advance and loading them to the GPS before I make a trip, that way I can include all of the great, out-of-the-way roads that I want to hit.