Anil Dash notes the double edged sword of Twitter's transience:
Perhaps the most important psychological innovation of Twitter is that it assumes you won't see every message that comes along. There's no count of unread items, and very little social cost to telling a friend that you missed their tweet. That convenience and social accommodation is incredibly valuable and an important contribution to the web.
However, by creating a lossy environment where individual tweets are disposable, there's also an environment where few will build the infrastructure to support broader, more meaningful conversations that could be catalyzed by a tweet. In many ways, this means the best tweets for advancing an idea are those that contain links to more permanent media.
He's right that the guilt-free nature of Twitter is a delight. You can go a few days without checking it and it's "okay." But the downside of such a set-up is captured in Anil's post title: If You Didn't Blog It, It Didn't Happen. This is not just about the 140 character limit of Twitter (though obviously that's a major impediment to conversations). Tweets come and go, and if it's gone, it's (basically) gone. As evidence of this, I'd link you to a "debate" David Frum and Virginia Postrel had last week on Twitter about health care policy, of which I read bits and pieces, but alas, it's too hard to find and impossible to reference in full.
Blog posts are easily linked-to, archived, tagged. Lengthy conversations can ensue in the posts and comments section. And an RSS reader efficiently captures everything (though it could do a better job at tracking multi-blog conversations). I'd give up Twitter without much of a fuss, whereas you'd have to fight me to death to take away my RSS reader and blogs. I'd miss Twitter, to be sure — it's fun to see an on-going stream of gestures from people I care about. But it's not core to my intellectual experience on the web.
Stowe Boyd concludes on a philosophical note:
Lurking behind Anil’s practicality are the more philosophical issues of time and transience. Yes, we don’t need to retain every tweet ever read or written. We can accept the fast and furious impermanence of most tweets, and the up tempo pace of the Twitter bloodstream. But we want to also operate at a slower pace, dealing with deeper and abiding interests, ideas, and connections. We need to be able to shift tempo without missing a beat.
Here's my post on the evolving uses of Twitter.