Castlevania is not produced or developed by Nintendo; they have nothing to do with it. Metroid and Mario games, of course, are, and their entries on the Wii have been excellent by most estimations.

Well, it seems like you have absolutely no idea what you're talking about.

I could "go into a lengthy diatribe about" the many brilliant ideas in Wii Music, but instead, I'll respond to your vaguely sketched out MIDI criticism, borrowed by none other than the IGN reviewer who stated far in advance of the game's release that he was dying to tear it apart, and who also posts those rather juvenile rants about how Nintendo doesn't make enough Zelda games, etc.

This will, however, require some quick digression into the question of the point of this game. This isn't a game with pre-recorded tracks that you simply try to play through in glorified Simon-Says style, as in Rock Band styled games. Instead, the entire objective of this game is to take a familiar tune and rearrange it into something new, or take it into a musical domain far removed from its original settings.

For the uninitiated, here's a very quick rundown of what you do:

1. Pick a song from the list
2. Pick an instrument to play, and a role for that instrument. This is crucial; if you pick, say, a cello, and assign it the role of "harmony", the notes available to you as you play through the song will be chosen in a roughly contrapuntal relation to the main melody. If you choose the role of "chord", the cello will be able to play chords following the basic harmonic progression of the song. Or assign it to "bass", and so on.
3. Play through the song using the chosen instrument in the chosen role. Here's the interesting part -- while the specific notes played will be drawn from the combination of instrument and role, you can play the instrument however you'd like, holding out notes for suspensions, syncopating the rhythm, adding fills and sectional variations, etc. Each instrument also has various advanced controls, allowing you, for instance, to double pick, mute the strings, or bend notes on the guitar. Of utmost importance to the quality of your performance, however, will certainly be your restraint and control of dynamics -- here Wii Music's controls truly shine, picking up movements from the slightest tap on the piano's keys to a resounding chord.
4. Now restraint and style becomes even more important: you continue to perform instruments in chosen roles until you fill all six parts of the song, overdubbing with your previous parts to create your own rendition of the song.

The possibilities are endless, and here's why, at last, something like MIDI is necessary for this game: you can't use prerecorded parts like other rhythm games, and you need to allow the user to do anything with the notes played, with dynamics, bending, and other touches depending on the instrument.

Perhaps they could have used even better MIDI voices, but the actually sound very good if used properly in a creative arrangement. Here are three very different videos, from three different authors, created with Wii Music: surely there is no mistaking the MIDI roots if you listen closely, but overall the sound is amazingly good for a game that is so open-ended.

Frere Jacques Every Breath You Take Sukiyaki