Are there any webmail providers out there that support PGP? GMail never will since it needs the ability to read your email to show you ads.
It could still read your email after decryption and show ads based on that, it would be easy to do with JS. Search would be broken though unless they encrypt each word separately, but that would make your inbox size grow quickly, still doable.
I can imagine an extension for Chrome or Firefox that will not be updated without my knowledge and has been peer reviewed like gnupg.
But yeah, like you said, if you're only checking emails on machines that are secure and in your control, you might as well just install Tbird/Enigmail. And if you're at the library or a computer lab, you can't trust the client to be secure.