More and more people are carrying cellular phones, either to do business, or for self-defense. A cellular phone is one of the greatest weapons for self-defense against crime. This is the information age.
A phone should be something we wear, not something we carry. If you're being mugged, you don't want to have to ask your assailant if he/she minds waiting for a few minutes while you dial 911.
Furthermore, there are times when you are not sure whether or not your life is in danger, and you don't want to dial 911 every time a moving shadow is cast on the ground from behind you, hence you would be better off merely being in touch with someone. It would be far better if you just happened to be talking with a good friend, and casually mentioned your location now and again, and what was happening around you. Ham radio enthusiasts have been "ragchewing" while walking down the street for years, and doing it at zero cost. And striking up a casual conversation helps to ward off would-be attackers as well. Thus whether you're wearing a ham radio voice transmitter, or wearing a cellular phone, having it integrated into your body in a comfortable way is a big advantage. A `personal safety transmitter' might also be capable of transmitting live video.
Computers are now something we carry. Reading your mail while standing in line at the deli is a whole lot easier if you're wearing the computer rather than merely carrying it.