Given how all manner of languages, libraries and frameworks tend to massively sprawl over time, keeping the growth of core components down is a good idea.

First, it's a very nice new streams interface. Second, the core functionality can't change that much at this point if people want their old code to work. Read the discussion of streams2 and see how much thought went into avoiding breakage. http://blog.nodejs.org/2012/12/20/streams2/