You would just need to make sure to send your private posts only to others that use trustworthy clients.

And how would you make sure of that? Is there a field in the user data that says: "419 I speak the protocol, but I don't give security grantees - fool you for talking with me?". Is there planned an easy UI for showing you which of your contacts are part of the secure network, and which ones are backdoors into the network?