The SD card is so that people who want extra memory can get it, and so those who don't want it don't have to pay extra. Also, to prevent piracy, you cannot play games off an SD card, you have to copy over to the Wii, which does take upwards of a minute depending how large the game is. You can copy games to any Wii that has the Shop Channel memory of previously downloading said game, so if your friend and you both have downloaded Super Castlevania IV, you can copy that game from the SD card to either Wii.