The usefulness of a system like this would be where the police have a suspect cornered with their weapons drawn, but the suspect refuses to comply with instructions. If the suspect makes a suspicious move (e.g. as if he is reaching to pull a gun from a back pocket) the police will shoot immediately, since they would not have time to react if they waited to see if he was pulling a gun or not. Right now, the human system provides a 100% false positive rate. A weapon system that incorporates this detector would allow the police to pull the trigger when the suspect makes the suspicious move, but delay the firing of the weapon until the system detects a gun. Even with a 16% false positive rate it would save lives.

That said, I don't think police would want it if it might increase the risk to their lives.