A question was asked on the FAQ:
"Does taking the engine cover off help the engine cool off or anything at all beneficial? Is it better to leave the engine cover on for protection (I'm assuming that's its purpose)?"
From what I have heard, it may help cooling a bit, but it is very minor. If you continually drag race, removing it may help prevent heat soak a bit longer, but no guarantees. I doubt you would notice in normal street driving.
It is there partially for protection and partially to make it look nice.
-Protection: To protect the coil packs and injectors from contaminants and water (although keep in mind uncovered you are not really going to have a problem unless you go mudding in your V)
-Look nice-Hey, itís a caddy, it should look nice...
Some people have commented that valve noise is higher when they remove the cover.