I see a few threat models where its useful. The government can subpoena your emails on gmail, but I think they might have a more difficult time convincing goog

I see a few threat models where its useful. The government can subpoena your emails on gmail, but I think they might have a more difficult time convincing google to change their javascript to add a $.post to steal your decrypted emails. Also, these attacks rely on the user actively logging in. So if you are in jail or dead , your data will be safe.