I would say that they are really needed. I ran without the small panel on my Celica that covered the bottom of the oil pan area (for the same reason you mentioned, it made oil changes quicker). After two changes, I noticed rust forming on my oil pan bolts and that everything under there had become salty. I waited until I had driven a few times in the rain to wash everything clean and then reinstalled the panel.
On my Rx7, the panel is needed to force air to go through the radiator. Without it, the car overheats at speed as the air can hit the radiator and flow underneath it. Obviously this isn't a common situation, but it's another reason to keep it if it covers the bottom of the rad area.
On my Rx7, the panel is needed to force air to go through the radiator. Without it, the car overheats at speed as the air can hit the radiator and flow underneath it. Obviously this isn't a common situation, but it's another reason to keep it if it covers the bottom of the rad area.