My first thought is that the Wii already has DLC via VC and WiiWare. Also, some games, such as Mega Man 10 (and 9), has DLC as well. Even Mario Kart, a Nintendo game, had DLC in the form of the free tournaments.
However, it appears you are talking about DLC that is common on the 360 or PS3 (i.e., I pay 5 bucks and I get a new track, character, or level/map). In theory, I agree with you. I would gladly pay a few bucks for a new circuit in Mario Kart, or a new world in NSMBWii.
In practice, however, I think DLC has actually been bad for gamers. DLC is being used by companies to get more revenue, and it appears that some companies hold back content so that they can charge for it later (Capcom is particularly bad at this).
I fear that if Nintendo follows this path, then they will start releasing games with less content. Take NSMBW as an example. That game has a ton of content. 8 normal worlds, 1 special world, a 2-4 player co-op mode in the main game, and two vs. modes (which contains another world's worth of courses just for the vs. mode). Now assume that Nintendo follows the DLC model: my game would only come with the 8 worlds. I now have to buy the co-op mode (which is perhaps the greatest thing about the game), the special world, and the 2 vs. modes separately and after I had already purchased the full game!
I think Nintendo currently has the right idea when it comes to DLC: it is either free or they don't do it. I actually like the way Nintendo does there DLC: every couple of weeks I can fire up Mario Kart and participate in a new tournament, or how the developer of Professor Layton does DLC by offering a FREE new puzzle every week. (Interstingly, this free DLC helps fight the used game market that many companies complain about-- but this DLC is available even to the used game buyers, as it should be).