Thats exactly what Stephen is doing. He doesn't like it anymore so he's leaving. What makes it it a big deal is that twitters success is based on prolific twitterers like Stephen Fry using the service and using it well. At the moment people are discussing how twitter is "doomed" for many different reasons this new piece of information just seems to add to the pile of evidence that twitter isn't providing value to the right people and this threatens its future.