Shawn, I agree with you that Depressed Center Flats were much ado about virtually nothing. Only 14 roads owned the 139 cars.
OTOH, if a product requiring an FD needed to be shipped, such a car would have to be acquired from some place. So...given that many such products were produced in the more industrial northeast or midwest [ Illinois, Ohio, Michigan in this case ], such products moving to the west coast might be found on any major RR between the originating plant and the destination. Hence, FD's would certainly qualify for long distance off line travels. True, one of these things wouldn't be seen often...but they would be seen.
Not only seen, but when loaded they are often spectacular, which explains why railfans were fond of taking pictures of them. I have a beautiful shot of PRR F29 #70008 on the SP in Los Angeles in 1957 and see no reason why modeling such a load shouldn't be pursued just because it was uncommon.
