Most clients do not implement (correctly or incorrectly) PGP at all. Instead they delegate the PGP parts to GnuPG, and failed to handle the warnings that GPG emitted. So your logic could be applied to GPG and argued that it is broken.
That's a pretty common claim and has been for a long time. It's not just verification of identity, but verification of integrity that the encryption is supposed to enforce. Instead, it throws a warning for one case and will not decrypt for another. The binary client is broken, as a security tool (insofar as it fails to provide the security it claims). If the integrity of the message is compromised, why show a version of the decrypted message at all? There's no guarantee that's correct and has led to this.