There have been a lot of WiiWare games then interest m, but I don't buy them unless I am sure I will like them. Once you buy a WiiWare game you are stuck with it whether you like the game or not, and you have no way to sell it.
Most WiiWare games cost between $5 and $10. That's a relatively low cost. If you buy a retail game--even a used copy--and then sell it back because you don't like it it's going to cost you more than you would lose by purchasing a WiiWare title that you didn't like.
There are also rental services. My point is that I shouldn't have to buy a game just to see if I would like it, especially if I will be stuck with a game if it turns out to be crap. For those of us with a tight budget, even $10 is a lot to basically gamble on the game's quality.
As for demo discs, the demo of Psychonauts in Official Xbox Magazine convinced me to buy the game, so I was always disappointed Nintendo never did them. I guarantee that subscriptions to Nintendo Power would have increased if they included GameCube/Wii demo discs with it.