Amongst other things, I hear this quite a lot on my conversations around the subject, why decoupling Drupal, is not like you are giving away what Drupal is and overkilling with something you could do with other tools? What's the point?
Well, in this keynote talk on Design4Drupal Dries mentions something interesting. Everyone things on Mongodb or any other cool technology in order to build your api... but what about the backend, the administration of that api? Who is going to input and maintain the content there? Will you have to build something from scratch?
Well, this is where Drupal decoupled comes into action. Why not having something that is API first, ready and designed specifically to manage content?