The security promise is simply "this is better than plaintext email, which is what you'd use otherwise."
I'd rather have as much of my communication as possible protected from mass collection by my own government regardless of it's sensitivity.
Edit: Misunderstood your phrasing a little. Yes, ideally we would provide non-savvy users with trivially easy to use encryption strong enough to defeat hostile governments. I haven't seen it yet. I make no pretense of overpromising - I'd certainly expect a service such as I'm describing to prominently note that it shouldn't be used for material above a certain sensitivity and link to info on better options.
