The problem with that article is that the author assumes that the only purpose for javascript cryptography is so that no middle man can understand the content, not the server itself. Javascript cryptography in this context is a more difficult problem only because you must trust that the code that the authentic source delivers does itself not contain a backdoor to the information.
Being cautious is important but keep in mind that the goal here is to be a replacement for having plaintext, sensitive info in your email history and chat logs.
You raise a good point. I'm not against encryption in the browser as a rule but it does open up a whole new can of worms. Our approach is to be just good enough for most usecases.
