The shields help with mpg and road debris. On the two Mazdas I have, I have repaired one and replaced the other that was ripped off some point in its history prior to my ownership.
The parts weren't expensive and the only issue I had was one of the front attachment bolt mounts was ripped out of the lower radiator support and I wasn't going to replace all that for 1 mount when there were plenty more holding the shield in place.
Realistically, the car could probably do just fine without it...but the AC unit and belt is near the bottom of the 2.3 engine, so I feel the shield offers a little protection.
