I was using MultiTrack yesterday and trying out the access to the iPod Library. My initial view is that this is going to be a great feature if you keep your own tracks in iTunes and want to use them and record other instruments and vocals over them.
The only downside is if you have a lot of tracks in your iPod library. If that’s the case, you might find it quite hard to navigate to the track you want to get to.
The track isn’t used direct from the library. Instead, the app imports the track and pastes it into a track in multitrack, and I assume that this is the same for other apps making use of the feature.
For me this is a great step forward and I can see it could be useful for DJ apps too, although I don’t think they’ll be able to use tracks immediately from the library and will have to import them in the same way as MultiTrack DAW.