Wouldn't instant mashed potatoes be the equivalent of processed potatoes anyway? I can't imagine they would be nearly as good for you, with not nearly the fiber and vitamins.
I see nothing wrong with processing--as long as it does not significantly damage the nutrients or introduce dangerous additions. Most foods that most McDougallers seem to buy are processed: washing, trimming, straining, precooking (as with many frozen vegetables), milling and separating grains from the chaff (as with wheat and oats). Refining is a different matter.
If the instant mashed potatoes are made from whole potatoes, then the fiber content should be the same. Presumably the flaking process destroys some vitamins, but I know of nothing making them worthless. Besides, I don't eat potatoes for the vitamins, but for the starch.
My former source for instant mashed potatoes, Barbara's Bakery, seems to have closed up their line. Here is an example of pure instant mashed potatoes, that I hope will soon be available.
I don't drive. Transporting, even by cart, 30 pounds of roots and another 30-50 pounds of other groceries is getting to be too much of a challenge for me. I walk 3 miles round trip on a grocery run. Instant potatoes, in addition to whole tubers, lightens the load.