Metroid and Zelda are good to keep. They could rotate the games, but why bother. If people dont want a cube with a free zelda or metroid prime, then they can just go home and play their ps2. Nintendo is giving a good enough deal now. Now they should probably add a pokemon game to the freebie list when the game is released (pokemon collusium). That should boost sales.
As for giving away third party titles, do you think that other companies would want to give away their games for free? I dont think so. If they agreed then great, but otherwise Nintendo would have to pay the companies to give away their games, so that wouldnt be smart.
On a completely jealous note, I bought my cube at launch, so screw all you people that got a free game just for buying the best console.