This seems nice idea.
I haven't tried it out yet myself.
I think it's a mix of expectations of consumption and creation. It takes less than a minute to text a tweet, and it takes just a few seconds to consume one. If there wasn't a limit to tweet size (or if it was much larger), then on the creation side, what constitutes a quality tweet would be partially based on the length of the content. That would add another dimension that might put off a users incentive to tweet. For example, when tweets are 140 chars, tweeting "I had a great tuna melt and Joe's Deli" is perfectly reasonable. However, if the average tweet length was 10 sentences long, that sandwich tweet would seem less reasonable, and maybe considered 'a waste of a tweet'. On the consumption side, if tweet length is much longer, you don't know what you are getting into. It becomes an entirely different experience if someone can tweet "great sandwich at Joe's", and another person can ruminate on Nitche's ideas of the origins of self worth.