I think, to the average Joe, the engine cover serves two purposes.
First it makes the engine bay look "cleaner" by covering all the parts that can gather dust, oil, etc... Lots of potential car buyers feel the need to look under the hood even when they haven't the slightest idea what is is they are seeing under there. The cover just looks "better" to most people, my wife included. It is a barrier between the reality that there is one hell of a complex piece of technology under that hood. It can also be wiped clean way easier than detailing the block itself.
Second, the engine cover makes the dealership money. Here's how: Say a guy has a fairly limited mechanical inclination, few tools, and a small amount of experience under the hood of any car, but is very willing to try his hand at DIY car maintenance. Say this guy buys an FX used and is glancing through his maintenance schedule one day when he notices it's time to change his spark plugs. He remembers doing this a few years ago with his last car and decides to give it a go. He pops the hood and BAM, engine cover.

He can't even tell where the spark plugs are, let alone deduce whether or not he is up to the task. With a sigh, he decides to close up the hood and schedule an appointment with the dealer.
I've seen that happen first hand with a friend of mine, though not with an FX. I tried to explain to him how easy it is to remove the cover but the mere thought of what may have been under the cover ("all the wires and hoses") was enough to dissuade him from even trying. Dealer - 1, DIY - 0. All thanks to a big ass piece of plastic bolted to the motor. :frown: