That mod reduces the time for your FSD to reboot, not the spool up time. It's a counter to FSD reboot missiles (AKA Grom bombs).
I really can't see a use case for it
In which case you may be hit during the spooling phase again... and again.
Ok, that makes sense. I won't notice this unless my FSD gets reset.
Power hungry setups that want to keep the FSD disabled when hardpoints are deployed while still being able to retreat quickly.
Hybrid/shieldless setups that can expect the occasional PP malfunction, or that may need to repair a damaged/destroyed FSD (low integrity FSDs are faster to break, but also faster to fix).
FSD disruption has a significant immunity cooldown and none of the delivery methods for the effect are guaranteed to land a hit.