in fact, this could be generalized more and doesn't neccessarily have to be about hiding messages. We've all heard the discussions about using VPNs "for privacy" (i. e. for hiding your IP metadata), when it's really just shifting trust away from your ISP and towards the VPN supplier.

It always comes down to who the alternative party to trust would be. In the fictional dialogue, the alternative appears to be to not send the message. Which may or may not be the better option than to give Eve eavesdropping capabilities.