Why should one use MIDI , the Musical Instrument Digital Interface, an almost 30 year old protocol, in 2012?
Well, you could argue that MIDI isn’t necessary anymore and that you can do everything in audio, and with many software synthesizers and emulators there’s even no need of it to control external gear.
But the thing is, MIDI has one important feature that audio lacks: it is endlessly editable in musical meaningful ways. You can alter tempo, meter, pitch, timbre, intensity and can control lots of other parameters.
I often do this parallel with graphic design: if digital audio is a photograph, MIDI (midifiles, to be precise) is a vector illustration. Sure, you can do lots of photo retouch, effects and editing, but only vector graphics let you alter at will shapes, colors, and all elements of the illustration.
So, from a composer point of view, MIDI is still an exceptional protocol/file format to create music from scratch.For example MIDI is the cornerstone of any notation program on earth, including our beatuful musescore, the venerand and ever useful abc, and the king of notation systems lilypond, used in the project mutopia to typeset thousands of historical music scores (a terrific resource).