It's really tough during a launch to tell from the outside if a delay was made for quality purposes or for marketing purposes. Given how similar the Wii hardware is in architecture to the GameCube, I suspect that these are marketing delays. This isn't saying that the developers can't use the delay to further improve the games, but I would guess that if they needed the game to fill a hole at launch, it could have been ready.
Ian ... your comment about Nintendo viewing third-parties as competitors is kind of silly, because it's the other way around. Nintendo makes money regardless. Third-Parties only make money if their game sells, so I've frequently heard that the third party publishers view Nintendo as somewhat of a "hostile partner" . Nintendo could very easily decide for business reasons that they need to delay some income into another quarter, and ask a publisher if they'd delay a game, or improve it, or even cause it to fail certification. I'm not saying that this ever happens, but I could certainly see how there might be incentives towards it.
However, because of the royalty structure, it's in Nintendo's best interest to have as much sell-through as possible, across the board. So Nintendo might be willing to delay a game to make time for some royalties now, knowing they can use the time to improve the quality and ensure more sales down the road.