<3 the answers from the ignorant fan bois.
The Wii isn't mean "for kids", it's meant "for everyone". This includes kids, but also middle-aged parents and grandparents in addition to the "core gamer".
It's unfortunate that the third party developers puts out crap for games for the most part. But there are good third party games out there. In addition to good first party games.
There's equally as many bad games for the other systems too. They just hide it behind shiny grass, hair, and tears in the graphics. The install base isn't as high for PS3 or 360 as it is in the Wii, so there isn't as much crap out for those systems, but crap still exists.
It's just that the Wii has been as succesfull as it is, is the reason the the crap developers put out quick cheap games that few people will enjoy. If they sell 10,000 units, they'll profit, so it doesn't have to be a block-buster game for them to get their return on investment.
There are some games that are M-rated (Conduit, Madworld, etc.) that was created at a "risk" to see how they would sell once released. If they do well, the chances that developers will make good M-rated games meant for the core gamer will increase. If they don't sell well, then they take their losses and go home.
So if you want to see more M-rated content for the Wii, buy the ones that are out there. You have to speak with your wallet to let them know that that type of game is what you're wanting to play.