I use GPG with my email client, but when I'm away from home, and I receive encrypted messages, I can't read them in Webmail. It would be handy, at least for me, to be able to sign, encrypt, read signatures, and decrypt messages in Webmail.
I'll admit I suspect not many Webmail users use GPG, but I know there are at least some. If there was a feature available in Webmail, who knows, maybe some people would start using it.
If the Webmail team isn't interested in working on this themselves, this seems like the sort of project some CS students would enjoy.
See these email threads for some discussion about how this could be implemented: