midi files as practice aids

I am currently defining ways of making practice tapes using Muse score's mixer function. When I discuss this with choir leaders their response is "you are proposing a solution to a non-existent problem since we already have all the practice aids we need". They then supply me with or point me to a selection of midi files and mp3 files. I certainly don't want to re-invent the wheel.

The names of the files suggest that they are meant for a particular voice - thus SongName-T.mid or SongName-T.mp3. So I'm assuming that these tapes contain material of one of these types
1 Solo SongName-T contains the tenor part only
2 Enhanced SongName-T contains the tenor part recognizably different from the other parts which
also appear

I haven't listened yet to the midi tapes as I have yet to install a soundFont that I need. In the meantime Marc, can you briefly help. You pointed out to me that MIDI files are for playback not for notation. How about as practice tapes? The Choraline practice tapes are excellent ; each one contains the given part on the French horn plus the other parts on some contrasting instrument(s). Also there is a narrator who warns you when to come in by referring to bar numbers in a well- defined score. My intention is to provide an mp3 using the Mixer function to play up the tenor part and play down the others. So I think we would have the Choraline property except for the spoken cues. We rely on the Muse Score synthesizer rather than the French horns .

The pieces we work with are not available on Choraline so, as I said, I am thinking of rolling our own. Just one thing; I certainly don't want to re-invent the wheel. Is it possible for you to comment on the midi file as an effective practice aid. Are there ways in which it is inherently more effective or less effective than an mp3 based on Muse Score and mixer? Or does it all depend on just how the musical information is presented on the Midi file? I'm sorry of this is a bit vague. I really need some expert advice before I go and make a fool of myself in discussing the issue with choir leaders.
Keith