I'm a little late to be getting into this conversation but here is my 2 cents. PNW refers to the Pacific, hence Idaho is land locked and I would reject their inclusion.
When I lived in Idaho we referred to our area as The Great North Western Inland Empire which included WA and OR from the Cascades East to the Rockies. There was a (small) movement at one time, mostly talk, to redraw state boundaries to include Eastern WA, Northern ID, and possibly Western MT into a new state. I don't know if there was any talk of Eastern OR and Southern ID as another state. I have nothing against Idaho, I loved it there but the growing seasons, native plant variety, precipitation and elevations are quite different. When I moved to the WA coast 20 some years ago, I was tired of snow from Oct to June. We would accumulate around 7' every winter, have even seen it snow on the 4th of July one year and have seen temperatures get down to -36F. I figured in Western Washington I didn't have to shovel rain or at least it would be a lot easier.