Poor Kilgrave would find that while Jack Harkness could feel sympathy and compassion and actually often did, he was also just a bit ruthless. The things he had done and had seen caused him to scar up. His heart wasn't easily swayed.

"Turned you out? Did you suggest to them that they might do that? It's how it works, right? You want something, you express that want, and anyone around you that understands you fulfills that want?" Jack wasn't giving the pheromone knowledge away. Not yet. Not so easily. It wasn't a trump card, but a little with held information during an interrogation never hurt.