When I hear the "Third party games don't sell on Wii" talking point it's usually closely coupled with assumptions about Wii's audience. Usually they say either third parties don't sell because Wii is dominated by Nintendo fanboys or by casuals that only want to play Wii Sports. Right? Meanwhile all the REAL gamers are off playing on their 360 so OF COURSE they wouldn't buy Wii software. And then the lack of third party sales is used to support this.
But if the games are actually shit, and the reason REAL gamers aren't buying them is because they are shit, then what does that tell us about how REAL gamers would react to good third party games on the Wii? Nothing at all. It's a vacuous argument.
Most third party games on the Wii- like Boogie, or Madden Wii 08, or Carnival Games, or that whole pile of post-launch Ubisoft tripe- don't sell because they aren't worth buying. Attributing their bad sales to the console audience rather than their lack of quality is misleading.