It seems to me, from reading responses on here, that other than for US66, Historic US routes are more community driven than some state sanctioned body. We all know what US 19, or US 281, or SR (insert your state of choice) 37 is. They are easily identifiable on any map, paper or digital. Historic US routes are not easily identifiable in the field, rarely shown on maps, and end up overlapping most existing routes.
Thus a debate, for the travel mapping community, that has not been solved, and most likely will not come to a conclusion (unless the operation of "Historic US Routes" changes among state and federal DOTs)