I agree, I don't see how implementing the same thing as Twitter but with missing features (like posting via SMS) is supposed to draw away Joe Everyuser.
The reason Twitter is popular is because there's a large active community of users and they do a good job of enabling you to discover and follow people you're likely to be interested in. It doesn't really matter how "open" it is, any more than it matters if Facebook or Tumblr are open.
WordPress works with a distributed "install it yourself" model because people use it to power their websites and they want configurability and extensibility. With social networks like Twitter, they want community and baked in support with devices and other services.