Nope. I still disagree. All you would get is a Pokémon with flashier graphics. Graphics mean nothing to me. Heck, I still pull out my ol' NES to play the original Megaman!
The other thing as to why Pokémon are Game Boy games is because it is something that'd be good to play when a TV and a console system isn't nearby. Because you can save at any point (except for in battles), you can pull it out, play it for a few minutes, and then save and put it away.
If it were on GameCube, however, you'd have to be at home (for the most part, you could get a Mobile Monitor and battery pack, but that costs additional money). Then, you'd have to wait for the GCN boot-up sequence to end (which is slightly longer than that of GBA), wait for the GCN to read the data from the Memory Card, and the such.
Plus, GBA games are still cheaper than GCN games. Nintendo wants Pokémon to be as cheap as possible so that many people can buy the games.