On a diesel you don't want a light flywheel, especially on an engine that is not inherently self balancing like the 5cyl Benz. Four cylinder engines self-cancel first order vibrations by having two pistons operating on the same plane. Inline sixes cancel first order vibrations a bit better and some second order vibrations, while an inline 8 will cancel all first and second order vibrations. Since the 5cyl does not have any paired pistons, it needs the mass of the flywheel to smooth it out. Without this mass, you can get vibration problems up to snapping the crank.