Actually, there are uses for 3 dimensional scale. For example, if you wished to know what a 1/4 scale autogyro should weigh, or the what displacement of a 1/4 scale engine should be. You just need to remember to divide for length, width and height instead of just one of the planes of measurement.

If the full scale autogyro weighed 500 lbs., and you wanted a 1/4 scale weight, then you'd divide 500 by 4 for length, divide again by 4 for width, and once again by 4 for height, which would provide a 1/4 scale weight of 7.81 lbs.

Trivial, but perhaps useful someday.