One case where the combined mixing/mastering pays off is when analyzing what tracks work together in specific parts of the song. If multiple tracks compete about the same frequency ranges, for example mid-range, it will sound very muddy and its hard to hear the individual tracks. If possible each track should operate in its own well-defined frequency domain. Even then, too much is just too much, the listener can’t separate all the instruments playing.
This is typical for the producer, as we could hear all the nice parts we added together, as we know the track inside out. While for a new listener, they don’t have that background, so the layers of sound will just confuse them. Usually two-three main musical lines is most what they could easily separate — it also depends on the level of musical interest and talent to separate lots of various instruments playing together.
Now, there are sometimes cases where layers and layers of reverb/delay-heavy tracks is exactly what it needed. Ulrich Schnauss is a good example of a producer who could pull of such walls of sound. Even so, there’s a lot of careful planning that needs to go in already in the mixing stage in order to avoid a big mid-range mush of sound that just confuses.
Anyway, check out Ulrich’s web site — he has downloaded material for free there, too.