I took my 2004 Impreza in to Rust Check when it was a week or so old. The owner said the rubbery factory wheel-well edge protectors promote rust. He encouraged me to pull them off, which I did on the spot. They were just glued on, and came off easily. I will probably never know whether this was a stupid or wise move.
I drove my old, 1993 Impreza (now owned by brother-in-law) for the first time in a couple years last Sunday, and noticed the fenders are all rusting through. It was never undercoated. Don't recall whether it had those fender protectors.
Driving the 1993 again made me appreciate the difference between the 1.8 L and 2.5 L engines, as well as the better support from the 2004's front seats.