Hi!
A guy at Caltech has posted several midi piano accompaniment files for Bach Flute Sonatas (My Theme these days...)
I found it interesting and wanted to share:
http://expet.gps.caltech.edu/~asimow/midi/flutemidi.html

This is quite nice indeed! It's a bit like playing alongside a computer, but the MIDI accompaniments are at least precise and clear for those wanting to practice this core repertoire before performing with a real human keyboard player.

Yes, I find them useful in learning a new piece. Also, if you have any SW application that can input the midi file and edit it, you can easily change the tempo (slower or faster), to help your learning process. Once the midi is being edited, you can also change instruments, key, practically everything you want or need.

When I upload a file to my profile (I have uploaded a few), I always include the midi for this reason, as well as a mp3 for better quality.