You'd only turn it on if your sensing system had a positive detection for a missile... any such system has its own failure modes, and while it's true that a sensing system with few false positives is a mitigation to hazards to the pilots, you have to consider the system as a whole. Especially if heat-seeking missiles fired at cargo planes are rare (and from what I can tell, they are), you expect the majority of activations to be false positives.