Not a gimmick depending on their application. There's a reason all the stock parts have aluminum heat shields... To protect a lot of the nearby components from the cooking radiant heat of the exhaust. If they didn't do much it would be cheaper for the manufacturer to just delete them from the design.
Similar setups are used in jet engine housings to protect sensitive equipment from radiant heat and it works well. Standing near a jet engine, with a fairly thin square of aluminum hold it in front of your face and the heat is gone.
Aluminum reflects nearly all of the infrared energy that strikes it, so it offers plenty of protection from radiant heat. Though, it does nothing in the way of stopping convection.