This question is inspired by the fact that I've seen some gamecubes sell for under $10 on ebay recently.
Clearly the backwards compatibility on Wii reduces people's desire to play on the physical GC console anymore if they own both. Likewise, people who have a Wii but no GC have no incentive to buy a GC in order to play the GC software. Finally, it may be the case that the GC and it's prior audience aren't old enough/nostalgic enough yet to want to use an actual GC to play GC games - it's not as attractive a target for a collector yet because it's too recent to feel historically important.
As for me personally, I never bought a Wii so my only way to play my old GC games is still on the GC. I never had many games (just like 5 or 6), so I don't wind up playing it very much at all compared to my dreamcast.