Kylie, you have a PS3 and you're worried about Wii updates!?!
But yeah. I think Nintendo does it to throw off possible cheaters/hackers when there newest game releases. People who buy a game like SSBB or Mario Kart Wii can enjoy the game for a little while before cheaters come in an ruin it. I'm sure it has some anti-pirating purposes, too. I haven't come across to many disc based updates but they are usually quick and painless (but annoying, nonetheless).