First off, great dialogue on this. This is a tough one...I think twitter has significant value if used the right way...same with Linkedin and Facebook. It's like a river...you can jump in when you want to and jump out when you're not up for it. Just don't get swept up by the current.
Think about David Allen, or anyone else that has a significant number of followers. I believe David has approx 1 million followers. To me, that's an amazingly powerful tool to connect with people that have interest in what you have to say. Whether it's insight into David's personal life or tiny nuggets of information, he can reach 1 million people in less than a minute. He's building his brand daily and the brand of Davidco/GTD.
My final point is that it's all about content. If you're providing valuable content, then I believe it's a great tool. Same with anyone you follow.