Good point about rubber between tube and road. Given the dynamic shape of rims I suppose in some cases the rubber of a 32mm tire will feel heavy and the sidewalls will move whereas a 28-30mm with a thorn resistant tube may provide a better seal around the sidewalls. I suppose one could fill too much, e.g. 32mm tire with thorn resistant tube in some cases. In some cases the tire and tube combo may be so tight that you compromise the tire changing a tube or maybe even just riding, leading to more flats.