Edit: also email puts restrictions on your identity. You're foo at some domain. @foo lets you pick a unique name that's valid online once you're old enough to pick who you want to be, without the footnote of _domain_ attached. Sort of how we all recognize Paul Graham as "pg", which fits his style.
...which is good. I want to be able to choose who I trust with my ID-related information.
But why is that? Is it because people will abuse it and send you data you don't want (spam)?
The same problem applies to twitter ids, except that not many people are spamming yet (I assume).
I guess there's also a single-point to detect spammers, which might make detection easier, but I'd be surprised if that was enough to control the problem.