Do all cross-contact allergens have to be listed in the ingredients list?

Recent guidance to the food industry deals with the issue of allergens in food that are only present as a result of cross-contact during manufacture, and are not ingoing ingredients. In the interests of safety, the guidance recommends listing such allergens in the ingredients list, even though they are technically not ingoing ingredients in the final product. Is such a measure required to protect consumer safety? What are the wider ramifications of such a practice on food businesses making processed foods and on consumers?