While it's true that there are "natural" divisions within some individual states, most non-locals are unaware of the distinction and/or unfamiliar with where the boundary lies. So if we split a state into multiple boards, we'll probably have more instances of people posting to the wrong board.
Fully agreed with your points. If this was a board about geology or topography, I would support dividing up parts of states. And I'm not 100% sure what constitutes which region or another, and resolving personal and/or regional identification is not within the scope of AARoads, either.
As much of the conversation on these boards deals with "state roads", keeping entire states together in the same sub-forum makes a lot more sense to me, and I would imagine, to those outside our membership that are searching for information on the web.
Visibly and geographically, it would seem to make sense to put Idaho in with the Northwest, and merge as necessary. So, I support the staff's choice of forum restructuring.