Anything that traps water or salty mud is bad news, the plastic underguarding on wheel arches was probably responsible for adding 10 years to life of a car before corrosion took it ( mud used to cake up under wheel arches and every time it got wet the salt in it would be re-activated). After accidents police used to look for a pile of mud on the road to establish the impact point). Cars do not get mudflaps as standard any more - if they did (especially rear ones) it may prevent a lot of the cracked windscreens we get.