The person next to you in cafe can MITM a self-signed TLS connection just as easily as the NSA; and the NSA can probably MITM a CA-signed TLS session, since the U.S. government owns or has access to quite a few root certificates. So, "no self-signed certs" is really a measure to protect you from the lowest level of threat. Almost any attacker than can MITM http can MITM https with self-signed certs that you never verify in any way. Encryption without authentication is useless in communications.