I have it at level 5 on my miner.
The Prospect may be for gas harvesting, but the Endurance is specialised in Ice mining. And it gets another 5% reduction in cycle time (and reductions every added 5% is relatively stronger than the previous one!) so for the Endurance this skill is definitely worth it to get to 5 eventually.
For the Prospect the reduced signature radius means you can more easily tank rats with speed/sig tanking, which is pretty useful as you have no drones to defend yourself. The mining bonus is nice too if you want to use it for ore rather than gas.
Reduced sig radius, as Xeus already mentioned, makes it harder to scan you down with combat probes which could be really useful when you’re in a wormhole refitting to huff gas.
Reduced sig radius also means it takes enemies more time to lock your ship, so you have a better chance to get away before someone catches you at a gate/wormhole/gas site.
I agree, for a gas huffing Prospect the bonus is less significant than if you were an ore mining Prospect or an ice mining Endurance. But it’s useful nonetheless.