Sensor fabrication has never been Nikon's forte - their strength is in making bodies. Until the Exmor came along, they were behind on every count sensor-wise - the 5D2 sensor beat the D3/D700 sensor in most respects, with the D700 being competitive mainly because it had a great body, while the 5D2 was little more than a great sensor in a budget box.
At the same time, Sony is great at making sensors, but not so great at making bodies.
Hence, it's an obvious partnership.
And I agree with the point I think you were making: that if cameras with a superior sensors were really a huge difference-maker in image quality, then people with D7000s 

