I do use zi.ma to shorten long URLs in Twitter posts. But I've used URL shortening for ages in email and IM, too, for the reasons outlined by gsimerlink.
And it isn't so much any alleged charm of Twitter (as fun as it may be), as it is the measurable web traffic from Twitter I see pretty much every day I post a tweet on our company account, that keeps me interested in the service.
But, as you say, horses for courses.
--Torka









