A few scenarios for using group shield leech:
Faction farming. Perhaps you're trying to raise, say, Red Dragon faction to gain entrance to QAR - find an area with lots of low level Chavez to kill, and pop group shield leech with reckless abandon. Follow with one or two hits with your lasers and finish them off. In a nutshell: group shield leech allows the JE to 'keep the pace' with other classes during the faction grind.
Another situation: Say you're in a group with a couple tanks, (or a raid group) and a number of mobs have been pulled to where several of them are being taken down at the same time and being tanked by more than one player. Perhaps halfway through killing adds B and C, everyone jumps on mob A to help finish it off. By using group shield leech, you can effectively negate the shield recharge on B and C, ensuring no effort or firepower was wasted in the initial damage done to these mobs.
I'd have to disagree with you on this. While it's true that Jenquai in general have the best reactors and therefore the most reactor power, and item bonuses like Beam Energy Conservation exist, I see the JE's expertise as being energy conversion, not conservation. Our Antlia devices convert groupmate's shield power to reactor power. Our Enviro Shield converts weapon damage into reactor energy. We can even convert our own Jenquai-made reactor energy directly into a Progen-made reactor (well, once our Phoenix devices are unbugged heh). So to me, it's perfectly perfect for us to have shield leech instead of the plain-vanilla energy leech.
"Heck, anyone with a straw can steal reactor energy, right?!" - Jenquai scientist, powering up an Explorer-class vessel
Cheers,
Kohr