On a 123 the thermister which measures air temperature is located at the base of the through-the-dash plastic fitting. It's about 1/2 inch below the air intake grille. By the time the air transits the foam/pvc/garden hose tube, it has already been measured. So I don't think the actual material matters very much.
Why did MB choose open cell foam? Probably because it's lightweight, cheap, and easy to install.