Crazy thought (sad that I even have to think about it), but what if a gun(wo)man entered the room and no one is able to call for help or give the police his/her position via their phone since it is locked.
Sure, the odds of an individual getting attacked at these events is minuscule. But over time the probability of someone bring a gun into some concert is pretty high. When that happens and it transpires that people weren't able to use their phones to seek help, the class-action lawsuits could come. The obvious solution (to the organisers at least) would be to bring aircraft-level security into concerts (security patdowns, etc.).