The GameCube library is really unimpressive overall, but it's similar to the N64 in that a few exclusives make keeping a small selection of titles worth it, especially if you never bought a WiiU. However, the Wii is better than the GameCube in every way, including being a GameCube. If you mod your Wii, you can even enable GBA connectivity to play Pac-Man Vs. or have somebody ruin your session of Wind Waker with the Tingle Tuner or play 4 Swords, if you have a lot of GBA's and friends that like bad Zelda games. Basically, with a Wii as a GameCube, you lose the little used system link and the GBA player addon, but then to have a good time playing GBA you have to get a controller that actually controls GBA decently and those controllers are crazy expensive.
You gain so much by using the Wii as a GameCube that there is basically no reason to own a GameCube any more.