75mm is a touch under 3 inches. My STi has more clearance than that underneath, and Subaru engineers still found reason to put a front undertray and two center smoothing trays on the car. Later models also had a rear diffuser installed from the factory It was a pretty ugly little thing so I sincerely doubt they put it there for looks!
I'm not saying that rock crawler 4x4's should expect much benefit, but I am confident that vehicles sporting normal road car ride heights will benefit.
Interesting to me is the benefit side skirts play in keeping the air under there until it can do some good exiting the rear of the car.
Speaking of diffusers, apparently they also do some good at the trailing edge of front splitters. Here's a sample image:
