I think there's a bonus feature to having two hops in the middle. If the sender finds that the recipient never received the message, he immediately distrusts his first hop and perhaps publishes the knowledge. If the first hop wasn't the culprit, he either publishes the second hop's unreliability or takes horrible devious Slytheriny vengeance on them.

So, due to mutually assured destruction, neither hop wants to defect and risk losing a nice income source permanently.