I know the "drip rail" on the A-pillar at the edges of the windshield are to help keep the side windows clear in rain, snow etc. The purpose was to direct the airflow over the roof rather than around the sides. There is some aerodynamic/ anti-drag effect as well, as the forward facing edge is airfoil shaped. In the case of the revised "bigger" size- probably the old adage that the engineers determined that bigger is better.

The C-pillar added black plastic fin (splitter?) between the body and the chrome rail was to keep more foreign debris from entering the vents in the pillar.

As for replacing the trim, the rubber is available separately from the chrome part of the drip rail. It is glued to the chrome rail with one of those special MBZ adhesives. MBZ might not even stock the older size rubber and has possibly superceded it with the newer rail. (I know that MBZ is trying to reduce the number of replacement parts stocked as I recently purchased a new underhood pad for my 240D and the one I got has the aluminum heat shield pad attached for the 300D Turbo cars. The 240D's as such is no longer available without the heat shield.)

The chrome part of the drip rail is attached via pressure to the body. You need to use one of those nifty nylon MBZ trim wedges to gently tap the chrome rail off from the body. I'm sure you could use another improvized tool to do the same, but watch that you dont scratch the rail or the painted body underneath!!

Hope this info helps!

J.G.