One of my pet irritations with the old instrument rack was that audio outputs had to be activated one by one. Now it’s possible, in one go, to select several which then appear in the mixer.
If you use VSTis that allow you to recall complex presets (like Vienna Ensemble) with Midi and audio routings, this can save a lot of bother.
The main advantage of the new instrument tracks for me is flexibility. Now, an instrument track is just like a midi track that comes with a slot in the instrument rack. Instruments are no longer segregated into rack/non-rack instruments.