Robert Scoble recently posted a piece called "Why I don't use Google Reader anymore". And his primary argument is that since he gets all his updates through Twitter, he no longer has a use for an RSS reader. The problem with that argument is that he trying to compare of two different update methods as analogous.
Twitter updates are serialized. They come in order and one after another. As such, you have to read through the stream to get your updates. If you miss a post, you have to scroll down through the stream to read it.
RSS readers provide information in parallel. They poll for updates from multiple sources and provide that information in parallel. You can skip to whichever feed you want to read, and go on from there. You don't have to scroll through your entire feed list to see a single article.