Oh, I agree that public key management and verification in the web client could be useful (though it could also be subverted more easily than a local GPG installation and keyring). Integration with the key server network, links between contacts and public keys, etc., would be very convenient, provided you could trust it. You wouldn't be able to decrypt anything, but perhaps you only want to verify signed cleartext.
However, you'd still need GPG on your own system to send signed messages, and a local public keyring for encryption. Once you have that plus a browser extension like FireGPG, how much extra benefit would the direct integration bring?