Moray,
I understand that you're "right" in that rats are technically the larger threat, but it's not going to make me see it that way... If I were to see a poisonous snake around my property, and I had kids or animals I cared about, I'd kill the thing too, and either take my chances with the rats or meddle even more in the ecosystem to take on the rats via another method (cat, etc.)
I might be "wrong," but I don't care. I'll rely on human instinct over empirical data when it comes to my own home. Perhaps I'll pay the price and catch the plague, who knows? It's better than seeing a kid/dog bit, IMO.