The problem with shortened links is that you have absolutely no idea to which webpage the link is going. The link can lead to an authority blog, a YouTube video, a spam site or may return a 404 error page in the worst case.
There are some tools and browser extensions available which you can use to know the actual location of shortened links, but if someone is using the official Twitter buttons to tweet links, you don’t have to.
The official Twitter button uses Twitter’s own URL shortening service (t.co) to shorten the link of the webpage which a user wants to share with his Twitter followers. The URL is sent to the pop up box, the user can customize the message and then send the tweet.
But when the links are posted on his Twitter profile, the actual location of the URL is revealed automatically. That means, you can know exactly to which domain a link is pointing to, if someone uses the Twitter button to share a webpage.
Agree that the URL’s are abbreviated but this feature will reduce some clutter from your ever expanding timeline, you can skip some links just by looking at the tweet and don’t have to open the URL.
The disadvantage : it consumes valuable characters, but that’s fine because the URL will be shrinked depending upon the length of the title of the post.
As of now, the expanded links work only with Twitter.com and partner sites or desktop clients still show the t.co version. Here is a screenshot from my Tweetdeck desktop client: