So you obviously don't need the Arduino to generate some music, but you could build something completely different, like:
- Let the Arduino listen on the midichannels and if a high C is detected start your coffee machine - C from Coffee makes sense
- make a Midi echo device - listen to Midi and play the same notes
- let it switch on the light above your sheet music, after notes detected and switch instruments off after 1 minute of no notes.
Especially the last one is usefull imho, someone gets a phonecall, does something different and forgets the light and keyboard etc..
enough inspiration to get started?