What an amazing post! a complete discusion on undertrays and driving slowly, as far as I am aware road surfaces as not much differant that they where 20 years ago, still got the same pot holes in the same places, In the motor world undertrays for cheap end of the market cars are relativley a new thing, first coming in in luxury cars.
There are still a vast majority of cars out there driving around with out them, be it designed for but not on for what ever reason or just not in the design of the car, very little damage done compared to the amount of cars out there without them, my wifea Meg 1 Coupe has never had one for over 7 years of ownership driven many miles in all conditions from sand to snow to floods not a problem, for the past 30 years of owning cars only 1 has had an undertray my Meg CC, which I took off in the summer due to water pump and cam belt then service, lowers arm work etc and now lower mount the tray is around the back of my house, am I worried NO! infact an undertray can disguise a lot of problems
drips and in my case coolant loss, I never run over debri in the road anyway that is just plain stupid.
They offer as much protection as a bit of plastic can, most undertrays are thier to deaden noise, and make the car more slipstreamed underneath (F1 airodynamics etc) and also like all the silly engine covers we now have to make the engine bay look like it is something not to be messed around in, as my CC take all the engine cover off and hey presto it is a bog standard engine bay, put em back on and wow it looks rather impressive every thing hidden.
I would be a bit concerned about hard board going soggy in the wet and falling off, that can do some damage to your car and the one behind in the rear wheels whip some of it up, youe main problem at the moment id the crappy gravel the council has launched all over the roads when we had snow and has now not bothered to clear up, the amount of cars we now get in with stuck handbrakes is amazing, inside there are tiny little particles of this grey gravel they used, I can not understand why the garage never put it back though, we would have a real rollacking for that and to say it is th ea room with the others means they don't put loads back, amin worry there is if they can't be bothered to replace a tray or should I say many trays what else can they not be bothered to do. but this "I Can't be Arsed" attitude seems to be coming through stronger and stronger in many trades now some of our younger tecs are awful with it.