A Webpro news article gives 8 reasons to stop ignoring twitter, from a marketing perspective.
It looks at things like Twitter Lists, re-tweets as means of marketing.

Unfortunately my views on Twitter have not changed since I first played with it: it’s NOT a marketing medium. I iniatilly felt that it was a self-indulgent network for middle class technocrati. My view has changed slighly, especially following the Iran elections, where Twitter proved to be a genuine medium for promoting democracy.

Since then, I have seen some excellent examples of consumer feedback. However, that’s as far as it goes with Twitter. Someone has produced a list of 50 CEOs on Twitter, such as Richard Branson. It’s interesting when you look at the reasons they use the network. Connecting to their brands, colleagues and so on seem to be their primary motive NOT marketing or promotion.

It’s hardly surprising this is the case. Twitter is, by it’s very nature, non-commercial. They don’t make money! I do not believe that users enage with marketing on Twitter either. In fact it will probably drive people away from a brand. If I want to find out about a product, for example, I’ll use Google to search for it.

So what is the point of Twitter? I would argue that it is essentially an information service, and to some extent an entertainment service. Most people use it to keep up with information in their sector or interest. Occasionally we get involved in some of the more fun threads. And unfortunately I still believe it is focussed mostly to the middle class technocrati. Twitter does what it does very well. But ultimately it’s niche and its NOT a marketing medium!