DirectMusic was a fascinating platform. It's interesting that while we've gained fidelity (in game music) by using mp3/vorbis streams, we lose some of interactivity and power afforded by sample-based formats. There are some interactive effects, dynamism, and music tricks that games based on MIDI and MOD formats/platforms (such as DirectMusic) were able to pull off that I'm not sure we're likely to see again in mainstream/AAA games.
It would be interesting to see if an open source toolkit like this might provide fodder for an interested indie or two to explore cool/weird soundtrack ideas.