The Wii was built for motion play. That was the core hardware of the console. The PS3 and Xbox360 can't quite match up to it -- even with add-ons -- because that wasn't what the PS3/Xbox360 was made for. The core of their consoles was creating a 1080p HiDef, intricate entertainment and serious gaming experience. The Wii wasn't made for that kind of gaming, so... add-ons to make it more high def may end up making the Wii look cheaper.
I doubt that it matters whether the manufacturers had motion play in mind when they first made the system. The Eye Toy, which plays into Sony's Move, and Kinect add-ons are sufficient for bringing motion gaming to their respective consoles. Not to mention that with the few years since Wii came out, technology allows the two companies to possibly bring a better motion gaming experience.
I like the idea of the Kinect not needing peripherals but feel that they are necessary for certain games, ie those that need buttons.
In my opinion, Sony's Move can probably offer the best experience of the three since the glowing orbs will allow a greater tracking of the player's position in space (which the Wii does not do) and the controllers allow it to be played for any type of game that can be played on Wii (unlike Kinect).