Whipped one of mine off today whilst in the area, just to assess the damages. As predicted, rather obviously, it's being abraded by dirt and debris which is also acting as a moisture trap. This is incredibly plain to see in the pictures.
Arch liner dirt
This little pile is what fell from the liner as I removed it.
Arch liner dirt 2
Bottom left of this image you can clearly see a patch of damp dirt, along with my rust. You can also see the dirt and debris on the top of the arch liner to the right.
Thankfully, I have gotten off rather lightly with just some minor surface rust around the seatbelt mounting point, which I shall be treating shortly. Alongside checking and treating this area, I would also recommend pulling the liners off with some regularity (once a year or so, or whenever you clean the arches) to ensure the area behind is clean. I will also be adding a layer of Dynax UB once clean as well.