I removed the rear spring covers on my 4/4 back in 2002, when I replaced the rear springs. Eager to see what the result was like, I chucked the seats back in without replacing the spring covers.
I thought at the time that I might get road dirt coming up past the spring, but it just doesn't seem to happen, the rear bulkhead trim covers most of the hole anyway. The spring covers are still not fitted to the car, and I can't think of a pressing reason to fit them, they certainly impede rearward movement of the seat. A simple piece of leather or vinyl would be perfectly sufficient to replace them with if one didn't want to have the springs visible.
