Other than security ^shrug^ I don't see the advantage in placing the audio bits in user-mode. In fact it sounds like a way to make the audio sub-system laggy to me.
Edit: This would also 'fix' fast-user switching where music can continue to play while you switch users.
Humorously enough, the "stop playing on fast user switching" feature (and yes, we had that as a feature) was left on the cutting room floor. Losing that feature was it was my biggest regret in Vista.
And there are a LOT of really good reasons, not the least of which is the ability to remove seven different device drivers. Don't understimate the impact of that, it's actually huge.
Moving the audio stack to user mode also enabled a huge amount of flexibility in what we can do with the stack, some of which will come out in the video.