I use it to set user access to my company's servers with ansible automatically. I just have to set a list of github usernames and it generates a list of users with their ssh key access setup !
It's an attempt to solve the key distribution problem. By having you verify your keys on third party sites, a MITM or NSL attack (providing you with fake keys so your messages can be intercepted) gets a lot harder as you have to attack n sites simultaneously instead of one.
I also view this as information leakage. I keep some of my online pseudonyms completely separated, and stuff like that allows people to link them together, if I was not careful enough to use a separate ssh key.
Seems like this would be a good way to frame somebody else. Hack into a server, do some damage/steal files, and drop somebody elses public key on the server.
"But I didnt do it!" - Then why was your key on the server?
Something similar has been available on Launchpad for years. There's a tool called "ssh-import-id". If I want to give you access to an Ubuntu server, I might type "ssh-import-id kentwistle". This would fetch public keys that the kentwistle user on Launchpad has published over HTTPS and then add them to ~/.ssh/authorized_keys.
I don't think there's any reason that ssh-import-id needs to be Launchpad-specific.
Github leverages such content-type negotiation for other resources too: add .diff or .patch to commits or pull requests. There's a way to get git am compatible data too.