I think he chose the cattle example because he wanted to showcase that both sides benefited from his contracts:
Cattle is sheltered from predators and can fatten at a rapid pace at a less stressful environment than it could have on nature.
Humanity in general according to QB wouldn't have advanced that far on the same timescale without magical girls. And magical girls themselves could have their wish granted through them.
His case does break down in the actual scenario of the series though, where the incubator is trying to enlist a single girl that has huge enough karmic destiny to end up ending all human life on Earth, and in exchange would yield enough energy for QB to leave Earth satisfied with their 'quota'.