I've updated my post to be clear that I meant more efficient for Twitter, not the end user in that case. Also, my assumption is that people use this instead of normal URL shorteners in most cases.
Regardless, URL shorteners cannot be described as beneficial for users (except those who want to track clicks on their links). They make it take longer to get to the page. They are an extra point of failure. They are bad for users. That doesn't mean it shouldn't be done, but it isn't altruistic by any measure.
The middle man will not be skipped. It will be wrapped. Two redirects instead of one.
http://twitter.com/twitterapi/status/15741693182