i think that the first and foremost reason a calibration done with Audyssey or MCACC or whatever is the fact that when the calibration is actually done, there is no one in the room to make additional reflections and holding the SPL meter in the hand; this can make quite a difference in the resulting sound.

See: http://www.blu-ray.com/community/gallery.php?member=Gelli
or: Axiom Gallery