You never know when you may meet a stranded cyclist. Avoid rush hour rides. Plus, the exhaust is really unhealthy. Very useful in getting the attention of drivers and others you are riding with in an emergency situation. Another safe measure is Amateur Radio.
Most metropolitan areas have a repeater tower that also has phone patch capabilities.
The radios don't weigh much and can easily fit into the bullet packs. They're great if you have to ride by yourself, because you're never alone. It's also life saving if an emergency occurs, to one of the group or an accident involving autos. The only requirement is a FCC license. Simple but necessary.