Just think, though, if you go covering the entire underside of your vehicle ... what about when you need to crawl under there to work on something? Like, say, to do an oil change? That might suck, having to remove a bunch of panels or one big panel just to get at other things that've gotta be removed, as well.
Plus, it just provides animals yet another place to crawl up in there and live when the vehicle's parked. Next thing you know, you're driving down the highway and start smelling burnt hair because of the cat that crawled into the undercarriage of your vehicle and is getting baked by the exhaust...