I keep hearing the graphics of the Wii are a fair deal better than the Gamecube's. Sure, it's not X-Box 360, but it's above X-Box.
Furthermore, I don't know about you, but I certainly wouldn't want to emulate N64 games. Games intended for analogue controllers and keyboard controls generally don't mix very well. (Yes I know, I could buy a controller for my PC, but then I might as well spend the $10 needed to get Mario 64 onto my Wii, and it'd be much more legal too)
Plus, this is a good reset for Nintendo. It caught the public's attention, and that of those ever important third party developers. If it was indeed possible to just release the Wiimote and sensor bar for the Gamecube, people and developers wouldn't have paid nearly as much attention to it. Plus, even if it's graphically somewhere above the Gamecube but below the 360, it still means that developers can try some new graphical doohickeys on it that they couldn't do on the Gamecube, so I'd say the Wii's existance is well justified in my opinion.
Seishun da!