Miyamoto once said that he was ashamed Nintendo didn't come up with the EyeToy idea, so I don't think he is pretending anything.

As for MS, they obviously wanted in on Nintendo's game, but wanted to differenciate themselves enough, so went with the obvious alternative (voice and camera combination).

Sony, on the other hand, is the one that seems to have forgotten the Eye Toy ever existed; the only reason they use the camera is to circumvent Nintendo's patents obviously, and the camera seems to be used for detection of those glowing sticks only, themselves clearly a different take on the Wiimote itself.

How would the camera measure your weight?

Also, I would't expect the camera to be able to detect minor changes in your centre of gravity, i.e. balance.