Someone mentioned above that, in the event of a fire alarm and being separated from your group, being able to reconnect with them is important. Particularly for vulnerable people (whether due to age, health reasons, or being someone who might be targeted for violence or harassment), being separated from your group for a long time can be a safety issue.
being able to reconnect with them is important
As I mentioned in another comment, it's possible you've never taken someone with a disability or social dysfunction to their first-ever concert, knowing that it's partly enabled by the fact that you can easily reconnect if you get separated (by the crowd, bathroom lines, a fire alarm, etc.) Take away that extra measure of safety and security, and the majority of people can "just deal with it", but people with certain disabilities or vulnerabilities are going to have to opt out.
Personally, I'd rather work to be inclusive. Use social norms, not technological lockouts, to get people to put their phones away.
Sure, cell phones can contribute to personal safety; whether it actually does in a standing-room-only rock concert given the distribution of risks one is likely to encounter in that context is another matter. I have a hard time imagining one, except a medical emergency, maybe? Except that having people find security might truly be more expedient.
Besides, I cam imagine real harm. Everyone simultaneously receiving a false alarm that a ballistic missile is headed there way might actually cause Widespread Panic to come out and play an encore.