Strongly decoupling API implementation and, well, actual implementation, is pretty key when you start to evolve an application. People often focus on 'the design' like there is one perfect design for an application for its lifetime when in really it is about how easy the mass of code you have is able to change for the next feature/fix/change and not turn into a hairball of code. That perfect initial design where the internal and external objects are exactly the same generally works well for 1.0, but not 1.1 or 2.0 so strongly decoupling the API implementation is a good general practice if you think your code will continue to evolve.