Wouldn't that require the user to install the browser extension to use the service? That would lock out a lot of people that would find this service useful (eg. Tails users).
SO WHAT? This is absolutely the most aggravating thing about JS crypto advocates: they truly believe that bad engineering can be turned into good engineering by sheer wishfulness. It's important, they say, for people who can't install new software to have encrypted messaging; therefore, browser javascript cryptography has to work. No.