"That scheme is, at best, only as secure as the server that fed you the code you used to secure the key. You might as well just store the key on that server and ask for it later. For that matter, store your documents there, and keep the moving parts out of the browser."
This ignores the scenario of app deployment models like Chrome Packaged Apps, in which the JavaScript code gets downloaded up-front and then is only used locally. Since you don't re-download the code every time, you only depend on the security of the code once, up-front, instead of on a continuous basis. You aren't affected by server compromise (well, no more than compromise of your OS vendor, but surely you aren't arguing that we might as well send all our keys to Microsoft, Apple, and Canonical).
Also I feel that this analysis conflates security with access. You may trust a company to keep their servers secure from compromise, but want them not to have access to the documents when the government comes knocking.