It would be very wise of Twitter to offer a bulk export function - not because they have to, but because it's to Twitter's benefit to not have me thinking twice about whether I should post to Twitter or communicate in some other, more archive-friendly way.
Look at the 2.5 hour Facebook outage from a couple of days ago. If Facebook were to be hit by an airplane or asteroid, a seriously significant portion of 500,000,000 peoples' lives would be lost, if only just in photos. If my house were to have the same thing happen, 150 people would experience a somewhat lesser loss.
I have thousands of emails between close friends in Gmail that I would like to preserve, but no convenient system exists for archiving them away from Google. The point of archive is to have redundant systems on which to rely. Archiving my Gmail with Google is therefore not an archive since it's still only one system.
It's like leaving a tasty pork chop in the 'fridge at work and expecting the cleaning lady to not toss it over the weekend. I really think archive should be the responsibility of the user.
How about IMAP?
I haven't ran it in awhile but when I did it worked and was able to download my emails from the very beginning without crashing. Anyway, you might find it useful...
1. Yes, Twitter's real-time just like the phone, but it's not only real-time like the phone. You can revisit any tweet you like. They support viewing old tweets, they support listing old tweets. My only concern is that they place limits on how much they list.
2. Yes, we're responsible for our content, but the significance of services change over time. Tomorrow's Hacker News startup might be interesting, you might drop some content in, but most people aren't going to immediately create an offsite backup. Likewise, the tweets we made in the first two years of Twitter's life probably seemed worthless to us at the time, but now their value has increased. It just would be nice to have access to that.
This means that if you want a full history of your tweets, then it's your responsibility to maintain it. Would it be a good service to its users if Twitter maintained full history? Yes, but that's not your argument. Your argument is that it's a user's right.
edit: from Twitter's Terms of Use (http://twitter.com/tos): "This license is you authorizing us to make your Tweets available to the rest of the world and to let others do the same. But what’s yours is yours – you own your content."
You own your content. You are responsible for it.