Hate that idea. I don't want to have to share my email address, in fact that's a primary reason why I always use my open ID (which is unrelated) when possible. Providing email as a credential creates an implicit, if not explicit, invitation "Here, spam me." This is why I'm generally against using email as an identifier.
Other examples that all suck for this reason:
Apple IDs.
Windows Live ID.
Jabber.
Other examples that all suck for this reason: Apple IDs. Windows Live ID. Jabber.