Here's my engine sans cover.
When I removed the cover I found plenty of dust and oil all over the coil packs and wiring. If the job of the cover is to keep the coils etc clean it's not doing a very good job.
And the noise reduction is negligible (for the N52) from inside the cabin so it seems the engine cover is essentially cosmetic.
In my opinion it's a hindrance to being able to monitor the condition of the engine and have easy access. It also will prevent heat escaping from the valve cover, shortening it's life. I'm keeping it off and I'm going to make small cosmetic covers to cover the coil wiring and injector rail.
