The usual argument against JS crypto is that a compromised (either technically or legally) host could send a malicious version of the code to steal your keys/data.
A browser extension could in theory be used to verify the code against a signature or something to that effect, or it could sandbox the crypto routines and keys from the app itself (though that wouldn't prevent the app from stealing the decrypted data)
What needs to happen for this to become generally usable is for the crypto part to be built into the browser - that way you only need to trust your browser and not a somewhat random plugin, nor the .js being served from a particular server.
And it'll "just work", so ordinary people have a chance to use and trust it too.
Sure, eventually, however I'd be just as comfortable using a plugin from, say, the EFF as I would having it built into the browser. It would be a fine stepping-stone (similar to how Google Gears contained many features which influenced HTML5)
And as I mentioned, merely having the crypto parts and key management built into the browser isn't good enough because a malicious site could still trick you into decrypting data which it could then steal, even if the keys themselves are perfectly safe.
A browser extension could in theory be used to verify the code against a signature or something to that effect, or it could sandbox the crypto routines and keys from the app itself (though that wouldn't prevent the app from stealing the decrypted data)