"Long journeys" are a critical point in this thread. If you bring a driver out of his normal element (e.g. eating dirt cheap Chinese food that he knows well) and put him in a tourist trap like Mutianyu for the bulk of the day, it makes sense to help out on the meal front whether or not you join him. Apart from simple courtesy, I certainly want my drivers to have sufficient energy at the end of the day when they bring me back to town.