I think the fourth of fifth graf of this (poorly written, I agree) article explicitly calls out that there are reasons beyond a secure channel why you might want clientside browser crypto.

So, I'm actually going a step past the obvious argument against jscrypto ("this is an inferior way to build a secure channel"). Not only is it an inferior secure channel, but it's also an inferior and dangerous place to try to build crypto functionality even if you're using SSL/TLS to establish a secure channel.