As the original is made of steel and bolted solidly to the inner arch, it tells you something about the level of impact protection that Rover felt was necessary for the fuel pipes. After all, a stone picked up at 70mph and thrown at these pipes is going to have a substantial force behind it (although that does make me wonder why they shortened the cover....
)
I personally wouldn't be too happy about a shield made of anything less tough than steel (i.e. plastic, fibreglass, etc.) especially as this area is well hidden and any damage could go unnoticed for some time.