Recently it's helped me see that a large email marketing firm has been hacked as many versions of the same spam message have arrived in my inbox sent to specific email addresses. It also prevents some of the cross-federated identity detection some "services" like RapLeaf try to do.

However, it becomes a pain with Gravatar, single sign on and places where email addresses have a provenance.

On that last point, I recently signed up for Qwiki, using the email address qwiki@mydomain.com. I also decided to sign up my girlfriend as well. Long story short, she got an invite and I didn't. :(

If Qwiki used any social media tools to discover "valuableness" of a given user from their email address, hers would have had great provenance and mine would have nothing.