Wii Music songs aren't covers. They are synthesised as the game will fill the appropriate notes at any inputs given by the player. As such, Wii Music behaves as a real instrument, though a very easy to play one. This is very different from rhythm games, such as Guitar Hero, where, don't matter how good you are in the game, the music will sound the same, regardless of your score. In fact, Wii Music don't need scores as it is up to you to evaluate how well you're playing.
Perhaps because I have some familiarity with music, I never had any interest in rhythm games, such as Guitar Hero. Wii Music seems to be the one of the few music games available and thus I am not surprised many people don't get it. I look forward to see if it will be any good.