The thing about Wii Music is that you're actually NOT writing music. The music is there, you're just poking at it to change its shape based on what's already there.
Watch some music videos, yeah. That should give you a good idea of what the game can do. It isn't really about composition, it's about finding ways to stretch the music, and the algorithm that guesses at the notes when you want to do something unconventional.
If you think about it in terms of composition, it's quite limited. If you think about it in terms of "play," or "variation," then it starts to make more sense.