This is a great point. Much of the value I get from Twitter is getting a feel for how the community is thinking about various subjects. Many times someone tweats on an idea or links to a new article which triggers a discussion and gets more new ideas flowing.
See David's post to read more.
Twitter is for forming groups - communities of purpose. Communities of purpose may be adhoc. They may come together to solve a shared problem for a short period and then disband, often with overlaps, as they evolve toward the next purpose. And Twitter is exceptional at doing this - because of its architecture, because of the
fuzzy-edge nature of the way groups form, reform and evolve.
The open sharing of our metadata, in the form of 'status updates' or 'look at this conversation-starting link' or 'look who I'm talking to' kind of tweets help us find our right-now community of purpose and start a conversation within it.