So I can log in everywhere using the exact same username? This will make it SO much easier for the user data trackers to capture and aggregate all of the information they can about me. I think I'll take a pass.
It's also one that isn't very easy to solve, regardless of how you slice it. Most sites that let you create your own username still require an email address. If you're using the same email address, we're back to square one here. Persona absolutely does not increase your trackability, and by giving users at least the option to use multiple, different email addresses, it's better for privacy than, say, Facebook Connect. That's a win in my book.
It's also one that isn't very easy to solve, regardless of how you slice it. Most sites that let you create your own username still require an email address. If you're using the same email address, we're back to square one here. Persona absolutely does not increase your trackability, and by giving users at least the option to use multiple, different email addresses, it's better for privacy than, say, Facebook Connect. That's a win in my book.