The only time I sold some old Nintendo stuff was when I sold a few Super NES games, including Super Metroid. Yeah, that was regretful.

Since I'm pretty confident that the Wii's backwards compatibility is pretty sound, there's no functional reason to keep my GameCube, true...but unless I get much for it, I probably won't.
Still, it would be nice to spread some GameCube love and it may help to sell it. Or I could lend it out. After the PS2 came out, it wasn't too hard to borrow a friend's PlayStation to play Final Fantasy IX (I'd played VII and VIII on the PC so it was nice to finish off the third FF trilogy without having to buy a PSX) as well as a few other games, so by lending out my GameCube to people who never considered one, it may be of benefit to getting some Nintendo converts...or at least to make them more accepting of Nintendo.