MIDI files are super small in size (only a few kB) and could be really usefull as background music in games. With MIDI support we could have many tunes in our games, without having a huge file size. And, ofcourse, it has a bit of nostalgic value too, as games like DOOM and Commander Keen used MIDI as background music. And you probably could also use your MIDI keyboard in games.

Currently Flash doesn't support MIDI. I couldn't find much about it on the Googles, but I found this:

Imagine what creative developers could do if Flash could transmit and respond gracefully to MIDI. Now an online petition is urging Adobe to do something about that. You need an account on bugs.adobe.com to vote, but that simply requires providing your name and email address. ...

Petitions have worked in the past. Wright points to the "Adobe, Make Some Noise" campaign for enhancing Flash audio, and the previous request for USB MIDI Device support, which is now under consideration at Adobe.

Source: http://blogs.oreilly.com/digitalmedia/
2008/04/petition-midi-support-in-flash .html

I think everyone on Newgrounds should sign that petition a bit!