<i>Train somebody long enough to ignore those warnings and you’ve essentially made End-to-End-Encryption completely useless.</i><p><i>Encryption without verification is ineffective.</i><p>Opportunistic encryption (without authentication) is still a very valuable tool, as it prevents passive attackers from reading your traffic. To actually intercept encrypted-but-unauthenticated messages, an attacker needs to obtain an (often permanent) MitM position, which is not impossible but still much harder to achieve.