"The E is already set up with a great rubber floor, and soil and water resistant seats - one of the things that make an E an E. Why cover it up?"
I asked myself this same question until one day we had some major rain. I walked thru some fairly deep water on the way back to my E, which deposited a good amount on the floor. It ran over to the side of the floor where it meets the wall....just disappeared underneath the plastic wall panel. As I drove home, the water would run out depending on which way I turned. Bottom line is, I don't know where it's going, but it can't be a good thing for it to be draining into "parts unknown"! I know the OEM mats at least have a small lip on the edges that might contain a bit of that otherwise "free range" moisture!