From Social Media Today:

The most influential people on Twitter are either already celebrities, create their own content, or both. Who do you see most often retweeted? Major news outlets like CNN, the Wall Street Journal and the New York Times. Mashable. Guy Kawasaki. Robert Scoble. Of course there are many reasons these people are influential, but a very basic reason is that they are creating original content somewhere other than Twitter. They are most often using Twitter as a super-news-feed, and as a way to drive people back to their blog, web site, etc. (Scoble is an exception. He converses everywhere.)

There are a lot of wonderful reasons to have a Twitter account (mostly to promote your blog), but I think the main relevant point is that I can’t think of anyone that is taken seriously only because they have a Twitter account. Twitter is used to promote something else, and it can’t really stand on it’s own.