An arbitrary and mostly-irrelevant technical limitation is responsible for the 140 character limit. But, it turns out that limit or no, 140 character messages have utility. Forced summarization means that you can benefit from updates from hundreds of people simultaneously.
Compare Twitter to the Facebook feed (I follow a comparable number of people on both). In neither case can I reasonably consume 100% of the stream. That's OK. But: I am objectively capable of benefiting from a far larger percentage of the Twitter stream than I am of the Facebook stream. I go two or three pages down Facebook and I'm saturated. I could never in a million years pay attention to it all day; I dip into it maybe 2-3 times a week.
The fact that Twitter sort-of works for this broadcast-y use case means that is sane to build things on it, like news services, or conference CFPs, or recruitments for open-enrollment classes, or announcements for new blog posts.
There are clearly things Twitter has wrong, and hashtags and inband metadata are clearly at the top of the list. But that doesn't make Twitter silly. The Twitter team apparently didn't know Twitter was going to work this well when they started it. It isn't some crazy scheme to trick the linkerati. They're still catching up to what they unleashed and I'm inclined to cut them some slack.