I think case in point with this is the engine bay cover that a lot of cars have these days. To me these sorts of design elements are useless costs unless they exist for these two reasons: either to scare off the curious from ever seeing the full beast, or to make it annoying enough to repair for the handy person who after seeing the amount of labor required for what used to be a trivial repair in older cars, simply opts for taking it to the service center because they lack time or an engine lift or any proprietary tooling.
I can't imagine they reduce a significant amount. I've driven some more recent cars that did not have these covers and they weren't any louder or anything. Most noise comes from your exhaust, if you don't believe it just take off your muffler and see what happens.
