I know you can play GC games on the Wii, but there are many reasons why it is better to play it on the GC if you can. Among the reasons why that is are these:
1) The Wii takes a little longer to get into GC games mode, whereas with thr GC you only need to turn it on. But on the Wii you actually need the Wii remote and use it to click on the disc channel first. This can be annoying, but it isn't a big issue really.
2) The GC is cheaper and easier to find in stores, so it is much less of an issue if it breaks down compared to the Wii. The fact is, the more you use something the more wear and tear it will receive, and I'd rather the wear and tear be on a $79 system which is easy to find, rather than a $249.99 system which is perpetually sold out. I'll try to save my Wii for just the Wii games.
There's probably other reasons as well, but those are my 2 main reasons for keeping my GC and still using it. Another plus is that the GB player and other GC accessories won't work on the Wii, so if you want those features you should hold onto your GC.