hrst: Contrast decreases with increased exposure too. What other reason? The best saturation and contrast point is obtained for x at x @ mid tones at box speed (unless you want to sacrifice by exposing less and pushing etc etc and other non-standard things) as defined by the manufacturer. People not seeing this I would suggest haven't experienced it, and there is some other reason for their improper exposure.
marK: No, the layers affect the other layers, on top of which increased exposure raises the minimum density of the complementary colour of the subject/area/point/etc. There'd also be greater stray absorption.