The intro to service describes the plastic piece you are referring to as underbody paneling and as an aid to improve aerodynamics and thus fuel economy. They say it also serves as a splash guard.
My ex-boss (MB tech for more than 30 years) says its crap and it only disguises fluid leaks. He takes them all off and leaves them off.
He also claims leaving it off improves air circulation from under the engine. I just took one of mine off to see if theres any noticable difference.
I guess its your call whether you feel you need it or not.
81 300 SD grey/palomino 168K miles
84 190E 2.3 black/grey 64K miles (wrecked)
85 190E 2.3 maroon/palomino 92K miles
88 300E desert red/palomino 204k miles
(made to look like a '94-95)
92 400E desert taupe/creme beige 120K miles
(converted to 94-95 E420 looks)