This works if physical contact is available and parties are trusted (even the weakest literal reading of fourth amendment says "right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures," -- exchanging a key token counts as such).

Yet what of basic online communications where key-exchange is necessary and where we have very little say about the protocols involved?

You probably know what I was getting at. Public-key crypto is very important in today's world. If we can't use the current system, we need something to replace it. Pre-shared keys is for a different scenario.