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This controller plugs into the wiimote and is powered by it. The way they made the cord come out of the bottom of this controller suggests to me that they want players to sit on the wiimote for a more enjoyable playing experience.
The idea of having to plug the classic controller into the wiimote just seems so inefficient to me. Perhaps since it does plug into the wiimote there would be a plus to hearing the sounds that are made to play through the wiimote's speaker, but this is a minor feature that wouldn't be missed. Why doesn't Nintendo just make a new version of the Wave Bird and make it so that the receiver fits flush on top of the Wii, and instead of gray it's white and shiny, heck even put rumble and a speaker in it too! This post may not have came to fruition if the GCWB is easy to find in used games stores but they're a rarity now.
The reason for this is because people are still using them! I went to a LAN party not too long ago and a lot of people were playing Smash Bros Melee on their Wii consoles (before Brawl came out) and everyone was using GCWBs!
Think about it Nintendo.