In terms of real-world deployment, police issue ballistic helmets would not need to be worn all shift. Instead, the more likely scenario would see the helmet kept in the patrol car, making it available to officers in the event of an emergency response scenario or IARD. Many agencies are already implementing a similar strategy with hard armour plates but strangely not with ballistic helmets.
You don’t need to look far for examples of ballistic helmets saving officers lives. In the Orlando night club shooting back in June 2016, a SWAT officer was struck in the head with a bullet, his life was saved by his helmet (a Galvion Viper A3).Without the protection of a ballistic helmet, the result would have been fatal.