"A BCP is implemented as a BrowserID IdP with an additional discovery service."
Wouldn't that mean that the IDP then controls what ID-enabled services the user needs to use?
In other words, if I have a @gmail.com email address, then I am forced to use Google's bookmark/contact service?
Sure, my email provider could allow me to choose what providers I want to use for various things, but as a user, I would prefer that my freedom to choose doesn't depend on my email provider being nice and implementing a configurable discovery service.