I think the advantage Twitter gets from being archived is greatly overestimated here. How many people actually consider that when they decide to sign up for Twitter? I'm also skeptical that Twitter is an alternative to a traditional blog. The 140 character limit doesn't really let you replace a blog if that's what you want to do. I think most people who want to blog would have a blog in addition to Twitter. Likewise, people on Twitter who don't have a blog, probably just wouldn't publish anything at all in the absence of Twitter, so it's a net gain in that respect.

Finally, Twitter does have a corporate API, isn't most of it accessible through the open web any way? Any user's profile can be accessed through the website unless they've specifically restricted it.

And generally, I don't see the point in NOT archiving something if you can. Sure it's fine to argue that other things should be archived first, but if someone is going to archive something, why stop them? We only gain by having a more complete and accurate historical record.