We don't always know what will be important. You could have said Twitter was a useless blogging derivative. Yet it has proven itself a powerful tool for social change, evidenced by the virulence with which governments turn off access.
I'm not convinced it's actually that important. There's this feedback loop where the media treats Twitter as important and so celebrities and politicians (but I repeat myself) treat it as important and so on. But do real people actually care?
I think it is important because it's an imperfect but easy-to-use tool to get a quick-and-dirty sense public sentiment. (When governments realize that they'd do better to perform a MITM attack on twitter rather than shut it down, allowing them to craft whatever public perception they like, then Twitter's usefulness will drop.)
