In general, I mix with audio and not MIDI, but the question was about MIDI fade and there are times when it is helpful to have this in MIDI: especially when there is a large number of small movements, like swelling strings in and fading them out on many phrases. Also, I can't get pedal steel right without a simulation of the volume pedal and I don't find the audio fader the right tool for that job.
So, overall mixing for me is audio but I use MIDI to mimic the instrument's behavior.