We still pay for single lamp replacements. Never heard of doing what you describe.
Al is more expensive generally speaking because it is harder (as in the hardness of the alloy), and that tends to be less forgiving and fracture/tear when hit and damaged. It may take more force to damage it (though any collision has enough force to render his point moot), but when damaged it's less malleable and less repairable than steel. More often requires a panel replacement versus repair than steel. So to your example a backing accident may require a lamp replacement but rather than massaging the dented steel around the lamp, the entire aluminum section needs replaced due to the damage and tearing of the metal.
Additionally tools are kept separate for steel vs aluminum to prevent contamination and corrosion, and shops handling aluminum charged a surcharge to cover that. This predates even ford and goes back to the Audi and European days. It's less common now but still happens.
