Ed Bradley, studio manager for MMR developer Ignition Entertainment, told Eurogamer that "we've had to drop support for the GameCube controller as it's a very much 'at developers own risk' proposition and we're not masochists!" The context is not entirely clear, but our best guess is that, while a library of code for the classic controller may be provided by Nintendo, the burden of programming for GCN ports is on the developer and Bradley didn't feel like expending the resources.
Here's our worry: remember when Smash Bros. Brawl producer Masahiro Sakurai revealed that GameCube controllers would be compatible, so as not to alienate those who prefer the traditional -- dare we say classic -- control scheme? What if Nintendo's beat-em-up only supports the classic controller attachment? The cynic in us fears Nintendo would nix GCN controller support in order to sell a few more attachments. Not that it's a bad attachment, mind you, but we already have so many GCN controllers lying around that we don't want them to go to waste.
Nintendo Wii console
Nintendo Wii Remote controller