Using MIDI live is really a question of how much you use MIDI live in the first place: if you're just using it for playing back backing tracks then it may not make that much difference if you use MIDI or MP3's. Besides providing accompaniment, MIDI can be used to control lighting, mixers, harmonizers and more not to mention that it can be used to store data for outboard devices such as programmable EQ's. I like being able to control the mix and tempo of my backing live, which MIDI allows. Also, it's still a very cost-effective medium and you'll be able to store far more MIDI files than MP3 files on hard drives or floppies.

I know people who are using both MIDI and audio together for stage use by using a program like Sonar on a laptop. This allows them to have vocal and instrumental audio tracks provide backing while keeping MIDI for stage control of lights. But it requires a very healthy laptop to do this and personally I don't like having anymore expensive hardware at risk on stage than necessary.