Cell phones don't work everywhere, especially if you like longer rides that involve the countryside. I carry a cell phone, as well, but I think it would be foolish to only carry that in case of mechanical failure.

I was out in the hills one day when I came upon a guy standing beside his very pricy bike. He had a flat and no equipment to fix it. All he had was a cell phone and it didn't get any reception. Seemed stupid to me.