Sensible change
I think this makes a lot of sense for Twitter. Essentially they are now a messaging backend that gets called by everyone elses front end. Rail's power comes from the MVC focused approach that excels in building web apps.
I think to say Rails offers no value in building out Web 2.0 apps is just plain wrong. It is extremely fast and flexible when it comes to building web apps quickly. If you want to high throughput messaging architecture though - look elsewhere.


