These nets are nearly always a good idea for any rally car, short of your rally car having a $2700 to $9900 seat like what is now required for WRC cars.
You have to remember that in side impact crash events, there is very little time in which to slow your body down. The earlier the safety engineer can slow your body down in a crash the better (this is why we want tight harnesses.) These nets work to slow your head and your upper body in big side impact crash events. They redirect that crash energy to the nets mounting points. That means less crash energy goes through your body directly.
Of course there is always catastrophic failure... Nothing really helps you there... Still, these nets can and will save lives and reduce crash injury potential in what is the most dangerous impact scenario for rally car occupants.