Say you want to send bitcoins to someone on a forum like reddit, Hacker News, Facebook, or by e-mail, and they don't have a bitcoin address. I propose a public service which will accept bitcoins on behalf of someone, reaches out to verify their ownership of the recipient account, and allows them to clam the bitcoins and set up an address to forward their bitcoins to in the future.

Would this be useful? What are the downsides?