When I had lunch with the other Joe Nocera recently, he told me that he tweeted purely for fun. Sometimes he sent tweets to sports announcers or players to see if they would respond (sometimes they did). Mostly, he simply offered up his thoughts about his beloved Yankees. Tweeting, he told me, helped relieve the stress of his day job.
( Source: NYTimes.com | A Case Against Twitter )
Seems to be the tweeting benefit that far outweighs the costs.