most of the points i agree with have already been said...
But in a nutshell, the beauty of twitter is in its 'dumb easy' aspect of it and the fact that it actually doesnt even want you to hang out at its website. It instead aims to piggyback on whatever it is you happen to be doing, like make its tweeting so painlessly easy to do that you can do it while youre eating breakfast, or watching a woman get hit by a car lmao.
Basically twitter is pointless if you ask me but i can see why or how it might appeal to others. Particularly those that use the online medium in a very casual manner.
I think citricsquid or someone mentioned, but twitter wants to 'own' the social networking landscape, much the way youtube/google now owns the viral video landscape.
The web is always changing and the trend is to not force people to be bound to a single website but rather make your offerings as versatile and user friendly as possible and available to a wide variety of platforms, i-phones, blackberry, mobile, laptop, xbox live etc.