The DOJ didn't want everyone's mail at Lavabit. It wanted Snowden's mail. The DOJ will feed malicious JS to its targets, and innocuous JS to everyone else.
Yep. They would. Snowden should be using PGP on an air-gapped machine inside a faraday cage, right? This isn't for him. This is for the rest of us that are at most mildly interesting but don't want to give in to the surveillance state, or just want to send a password without blindly and completely trusting at least one third party. Isn't such a mid-security but easy to use solution valuable? The more PGP-armored messages there are (even if they could be broken with effort), the less suspicious any such use of crypto can be considered.
There is virtually no correlation between the people who need to have their secrets protected from hostile governments and the people who are savvy enough not to use applications that make security promises they can't keep.
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.