A very similar approach was recently proposed for making time zone boundaries more manageable, but if we implement it, it would actually have the opposite effect, increasing the burden on both mappers and data consumers:
Besides, such a representation would usually be fundamentally inaccurate. While it is the case that the U.S. is composed of “the several states”, its boundary is officially defined as a few linear rings. The boundary description for the contiguous U.S. makes no reference to interior states such as Colorado, but it does refer to rivers, distances, survey lines, and specific coordinate pairs. This is how the vast majority of boundaries are customarily defined in this country’s legal tradition, from the country all the way down to the parcel on which I’m standing.
It makes for a pretty good test as to whether something is actually a boundary: a boundary can be perambulated based on its description, if not in person then from an armchair. My initial hesitation about Connecticut’s “council of governments boundaries” was that COGs in other states are merely membership organizations. Eventually it became clear that what was being mapped was not the COG but rather the planning region for which the COG is responsible, which is defined as a boundary.
Maybe you can guess how I feel about boundary relations that consist of member_state members, such as NATO.
Just about the only situation I know of where a subarea structure might have some advantage would be the subdivisions of an Indian reservation. On some reservations, the local jurisdictions form a sort of “diamond inheritance”, to use a computer science term.
A given boundary relation could be a subarea of both the native and nonnative civil boundary relation structures. But there’s probably an alternative way to solve that problem that doesn’t incur the disadvantages of subareas. The status quo has been to define an alternative primary feature tag for the reservation as a whole, so logically we could also define a secondary key to use in parallel with the boundary=administrative admin_level=* tagging scheme.