I suspect, as others have alluded to, that this aspect ratio change is going to be done to (eventually) _simplify_ developer's lives: you'll have the 16x9 format to target for iPhone, iOS TV, and perhaps, in the future, iPad.
Apple has shown that they're willing to pay the cost in making changes now for simplicity further down the road, and this may be along the same lines.
This is a very good point.